What's a good place for info on Amsterdam and Amersfoort. Accomodations, transportation, dining, etc?
You can start with these:

Official Amsterdam site (just click on "english site")
Visit Amsterdam
www.holland.com

- How's public transport in Amsterdam?
Pretty darn good
Can it get me to Amersfoort?
Yes, very easily, by train
Can it get me around Amersfoort? Can it get me to "stuff to do" in the surounding area?
Yes, but there will also be enough people with cars...
- Some of you (Americans) mentioned using your cell phones there. Can I do this using a StarTac (no) or a Ericsson T28World (GSM, so maybe)?

The T28W should work (mine did in the US ), but make sure your GSM provider has roaming arrangements