Staying more than 3 months in Europe (Schengen agreement question)

Harry's experience with visas and EU countries has me wondering about my own trip:
Staying more than 3 months in Europe (Schengen agreement question)

I think I should be fine though. My first 2 weeks are in the UK, which doesn’t participate in all the Schengen arrangements. Plus I’ll be leaving the EU for a few weeks for Eastern Europe, then I re-enter the EU. So all should be okay.
Hmmm… I wonder when Poland and the Czech Republic officially enter the EU?