to start. However, the general guidesline for a requirement to file would be to add the personal exemption amount and standard deduction amount for your marital status. For 2010, that would be $3650 for the personal exemption and $5700 for a standard deduction for a single taxpayer (or $9350 total).
There are other scenarios that you'd have to be aware of, like if you make more than $400 from self-employment, you have to file and pay the self-employment taxes (15.3%), even if you don't meet the other filing thresholds.
Lastly, you may want to file if you're had income taxes withheld, so that those amounts can be refunded.