Kelli, I sent you an email, but I'll summarize here as it might benefit others. This has all been confirmed by an email from SSA.
Basically, any month in 2007 where you earned over $640 counts as a Trial Work Period (TWP). In a 60 month rolling period, you can have 9 of these months, (TWP), where you earn over $640.
During these 9 months of your TWP, you can make unlimited amount of income from work, and still recieve 100% of your SSDI.
Once you reach 9 months (and they don't have to be consecutive) they then look at HOW much you are making (whereas before they only cared if it was over $640 or not). For any month that you earned over $900 (for 2007), you do not get your SSDI benefits. The $900 is referred to as Substatial Gainful Activity (SGA)
However, if you have a month(s) that go under that $900, even after you've completed your TWP, then you are elgible for benefits.