Fixed SL5 build, but also other builds (please see comment below). Should be painless to integrate this. Also, please let me know next time you want to push a new NuGet build so that I can update SL5 binaries, thanks!

Microsoft recommends to use TimeZoneInfo instead of TimeZone for all recent
versions (starting from 3.5) of the .NET, plus this is the only class
available in non-mainstream flavors of .NET (like Silverlight),
so why not use it.
Also update the assembly version of the SL5 build of ServiceStack.Text.