Hi Megs,
I've spun your post off into a thread of its own, as it's a topic that could be useful to many others. And you're in something of a unique situation - at least for this year. (Recap for lurkers: Meg has a green card, having succeeded in the green card lottery last year. She made her first entry into the US late 2009, but is currently still living in London - if I follow her prior posts correctly.)
Normally, as a green card holder, you need to file a US tax return for 2009 if you meet the filing threshold for your filing status (single, married filing jointly, married filing separately, head of household). In your case, you would normally be treated as a "dual-status alien" for 2009 - i.e. non-resident for the period up until your "permanent residence" (i.e. green card) kicks in, and resident from then until the end of the year. If you're currently living in London still, it's going to complicate your tax status - unless you're claiming residence in the US from your first entry back last year.
There is a tax publication (# 519) called "US Tax Guide for Aliens" available on the IRS website
http://www.irs.gov/pub/irs-pdf/p519.pdf Because your situation is kind of unusual, you probably need to read through the guide to get a better idea what your options are, then come back with further questions.
For general tax information, whether you're a US citizen or a foreigner, the best source is publication 17 - also available for download on the IRS site
Forms and Publications
Cheers,
Bev