Creating filterable selectbox attribute using SOAP API v2

Your array should look like this:

$array = array(
    "attribute_code" => "testattribute",
    "frontend_input" => "select",   
    "scope" => "store", 
    "default_value" => "",
    "is_unique" => 0,
    "is_required" => 0,
    "apply_to" => "",
    "is_configurable" => 0,
    "is_searchable" => 0,
    "is_visible_in_advanced_search" => 0,
    "is_comparable" => 0,
    "is_used_for_promo_rules" => 1,
    "is_visible_on_front" => 1,
    "used_in_product_listing" => 1,
    "additional_fields" => array(
        array('key'=>'is_filterable', 'value'=>1),
        array('key'=>'is_filterable_in_search', 'value'=>1),
        array('key'=>'position', 'value'=>0),
        array('key'=>'used_for_sort_by', 'value'=>0)
    ),
    "frontend_label" => array(
        array("store_id" => "0", "label" => "This"), 
        array("store_id" => "1", "label" => "That")
    )
);

For some reason the additional_fields must be passed as an array of arrays. The lower level array must have 2 elements: key & value.
If you don't like this method you can extend the wsdl to be able to pass the attribute is_filtrable (and others) just like is_searchable.
You can do this by creating a new module. Let's call it Easylife_Catalog with the following files.
app/etc/modules/Easylife_Catalog.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Easylife_Catalog>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Mage_Catalog />
            </depends>
        </Easylife_Catalog>
    </modules>
</config>

app/code/local/Easylife/Catalog/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Easylife_Catalog>
            <version>0.0.1</version>
        </Easylife_Catalog>
    </modules>
</config>

app/code/local/Easylife/Catalog/etc/wsdl.xml

<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns:typens="urn:{{var wsdl.name}}" xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
             xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
             xmlns="http://schemas.xmlsoap.org/wsdl/"
             name="{{var wsdl.name}}" targetNamespace="urn:{{var wsdl.name}}">
    <types>
        <schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:Magento">
            <complexType name="catalogProductAttributeEntityToCreate">
                <all>
                    <element name="is_filterable" type="xsd:string" minOccurs="0"/>
                    <element name="is_filterable_in_search" type="xsd:string" minOccurs="0"/>
                    <element name="position" type="xsd:string" minOccurs="0"/>
                    <element name="used_for_sort_by" type="xsd:string" minOccurs="0"/>
                </all>
            </complexType>
            <complexType name="catalogProductAttributeEntityToUpdate">
                <all>
                    <element name="is_filterable" type="xsd:string" minOccurs="0"/>
                    <element name="is_filterable_in_search" type="xsd:string" minOccurs="0"/>
                    <element name="position" type="xsd:string" minOccurs="0"/>
                    <element name="used_for_sort_by" type="xsd:string" minOccurs="0"/>
                </all>
            </complexType>
        </schema>
    </types>
</definitions>

After creating this module you will be able to pass the attribute create array like this:

$array = array(
    "attribute_code" => "testattribute",
    "frontend_input" => "select",   
    "scope" => "store", 
    "default_value" => "",
    "is_unique" => 0,
    "is_required" => 0,
    "apply_to" => "",
    "is_configurable" => 0,
    "is_searchable" => 0,
    "is_visible_in_advanced_search" => 0,
    "is_comparable" => 0,
    "is_used_for_promo_rules" => 1,
    "is_visible_on_front" => 1,
    "used_in_product_listing" => 1,
    "is_filterable" => 1, 
    "is_filterable_in_search" => 1, 
    "position" => 0, 
    "used_for_sort_by" => 0
    "frontend_label" => array(
        array("store_id" => "0", "label" => "This"), 
        array("store_id" => "1", "label" => "That")
    )
);

If you want to use WSI Complience you will need an additional file app/code/local/Easylife/Catalog/etc/wsi.xml

<?xml version="1.0" encoding="UTF-8"?>
<wsdl:definitions xmlns:typens="urn:{{var wsdl.name}}"
             xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
             xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/"
             xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
             name="{{var wsdl.name}}"
             targetNamespace="urn:{{var wsdl.name}}">
    <wsdl:types>
        <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:{{var wsdl.name}}">
            <xsd:complexType name="catalogProductAttributeEntityToCreate">
                <xsd:sequence>
                    <xsd:element name="is_filterable" type="xsd:string" minOccurs="0" maxOccurs="0" />
                    <xsd:element name="is_filterable_in_search" type="xsd:string" minOccurs="0" maxOccurs="0" />
                    <xsd:element name="position" type="xsd:string" minOccurs="0" maxOccurs="0" />
                    <xsd:element name="used_for_sort_by" type="xsd:string" minOccurs="0" maxOccurs="0" />
                </xsd:sequence>
            </xsd:complexType>
        </xsd:schema>
    </wsdl:types>
</wsdl:definitions>

catalogProductAttributeEntityToUpdate seams to be missing in WSI complience.