I cam across this page on Wikipedia the other day...
Up to now I've thought the new ToHS standards were a good thing... Should I be concerned about this in my boards?
I don't know whether you should be concerned about this in YOUR boards, but I'm concerned about it in commercial equipment that could fail in assort catastrophic ways. There are assorted industries who know what they are doing in the reliability business (ie telcos) that are forbidding lead-free soldering and such (I'm not sure whether they insist that components be tinned with lead-based solder as well, or just that lead-based solder be used
for assembly...)
Solder with more silver is resistant to tin whiskers.
More expensive though.
For personal projects its fine. You probably wont have a problem.
I'd be a bit grumpy though if I was in the middle of surgery when a whisker decided to short circuit though. 
Personally I stick to lead. Cheaper and more reliable.
The environment is already fucked. A little more lead wont hurt. ;D
Tin whiskers are a problem. That's why when you design something you expect every bad thing to happen and have multiply redundant safeties.
To remove the tin it one company I worked for shipped it out. The reason is that the solder has to be regularly tested and there are companies that specialize in removing the tin. This was for Class 3 soldering to mil-spec.
Conformal coating can help.
From the above website Paralene seems to be somewhat resistant. From experience it's nasty vicious stuff to get through to repair stuff. It slowly boils off if heat is applied and sucks the heat away.
It's very expensive to apply and only used on assemblies needed extremely high reliability.
Conformal coating isn't perfect esp. in extremely high humidity according to that article.