It will no doubt be harder to find a job in C# as a java dev. But it's not impossible. To that end, insert the usual advice about working on open source projects in the desired platform so you have some demonstrable experience. Often, Java for 5+ years with 1 year of .net will open many of the same doors as 6+ years of .net.
I've known people who've made the switch. Most are successful. A few keep their feet in both afterwards. A few never look back. A few hate it. The last group in my experience is mostly die hard MS haters to begin with and never gave themselves a fair chance. Most folks still prefer java but that's because they have more experience in java and it's easier to do what you know more thoroughly.
I did the switch in 2003 from a VB6 and java background. It was at that time relatively painless. Now, I think they've had a while to walk down their own roads and the difference is wider, but nothing dramatic. Things like LINQ will pleasantly surprise you I suspect.
I would also say that anytime I've looked at other platforms, I've learned other perspectives that have helped me on my platform of choice. Poking around in other technologies is nearly always rewarding for me.
Finally, I wouldn't recommend it unless you can be happy with the choice though. As I said, the people who fail in picking it up in my experience are those with strong anti-MS feelings. If that describes you, I'd do some soul searching.