(for 2009 – don’t have the info for 2010 yet) If you are single and under 65, you don’t have to file if your gross income was UNDER $9,350. Married, $18,700. You can download the 1040 instructions (filename i1040.pdf) and get this info, and it’s probably on the irs.gov website, too. You may want to file anyway if you qualify for certain credits, like the earned income credit, the making work pay credit, and some others.
Rules are different if you are a child or a dependent of another taxpayer, or if you have other taxes you have to file (like self-employment tax, or taxes on an IRA or something).
I’d say get the 1040 instructions and just check it out to see what applies in your case. You can get them here, and search for 1040 under Form and Instruction number, then download the Inst 1040 document.