There is no support in either CF or the full framework.
OpenNETCF has serial classes which can run on both platforms.
There are some samples here:
with a write-up called "P/Invoking Serial APIs in the Compact Framework" by Chris Tacke here:
For the full framework, John Hind has an excellent article called "Use P/Invoke to Develop a .NET Base Class Library for Serial Device Communications" here: