becker,
I did a little more looking about what deductions dependants are currently allowed to claim.
Here's a link to standard deduction topic on the IRS's webpage for those interested:
http://www.irs.gov/publications/p501/ar02.html#d0e5522
It states that even people claimed as dependants by others are eligible for a deduction of between $850 and $5350:
If you make $550 or below: your standard deduction is $850.
If you make $851 to $5050: your deduction is the amount you made + $300.
If you make $5051 or above: your deduction is $5350
So I was mistaken when I though that those claimed as dependants are inelligible for receiving this deduction.
Also of note: my discusson has concerned only federal income tax. Other taxes, such as social security tax (around 7.5% on earned income for about the first $100,000 you make), medicare tax (roughly 2%), and state income taxes (California's highest bracket is around 9%, most states are less, some have no income tax*) will probably result in you paying some amount more than the described federal income tax brackets. I believe that deductions won't do anything about social security other federal taxes, and the rules for deductions on state income tax returns will vary by state.
*more info on state income tax rates (there are links to all states at page bottom):
http://swz.salary.com/salarywizard/layouthtmls/swzl_statetaxrate_CA.html
For your second question, why some people prefer to stay under $600: As I said, all winnings should be reported to the IRS. But if Trainn only needs to inform the IRS that they sent you winnings if they send you $600 or more. So ifi you stay under that amount, Trainn doesn't notify the IRS and it's up to you to be honest and report how much they sent you. The IRS has no way of knowing that you got winnings if you received under $600, because they didn't recieve that form which tells them to expect that you'll report the winnings. I'll leave it up to you to guess how many people actually report winnings less than $600.
Example: Suppose somebody doesn't report these winnings, and let's say pays 20% total taxes (fed income, social security, etc). If he gets $550 of stuff and don't report it, he pays $0. If he gets one more game, putting him at $610 worth of stuff, the IRS will know he got that much and he'll need to report it. So he'll pay $122 (20% of $610) in taxes all because of that extra $60 game he received. Combine that with the fact that, once you're doing that many offers, you've probably depeleted the $1 ones and are doing things in the $8-$12 per offer range, and you can see why staying under $600 makes sense for a lot of people.
If you're a student without income (or very little income) it looks like your deduction will erase your winnings for purposes of federal (and possibly state) income tax. But you still might pay some social security and medicare taxes... so you still might wind up paying around 10%. At 10%, going from $550 to $610 winnings goes from $0 to $61 in taxes. So that final game wasn't worth it. If you're going to get a bunch of other games in this calendar year, and push your winnings to around $1000 (with about $100 in taxes) it could become worth it.
The information I give is just a rough guide for those curious about how the value of stuff they recieve from Trainn affects taxes paid. It's not legal advice, and shouldn't be considered legal advice. I'm not a tax attorney, or accountant. I'm just sharing my thoughts.