Ezra,
The good news is that no, you don't have to wait until the end of the year to do your Roth contribution. Congress had the foresight to realize that this might be a problem for some people, so they will allow you to unwind what turns out to be an "excess" contribution, as long as you do so by the due date of your tax return (including extensions if you file an extension).

So you can go ahead and make the contribution for 2008 now if you wish. If you sit down in the start of 2009 and find out that your income was so high that you actually weren't eligible to contribute, you can withdraw the excess from the account (be certain to inform the Roth IRA custodian that it should be classified as a withdrawal of an excess contribution).

However, there is a minor catch to this. Technically, the tax code requires that you take out your original contribution, PLUS/MINUS any attributable growth or losses on your investment. So if you put in $5,000, but your account goes down 10%, you are only supposed to take back $4,500. If you put if $5,000 and the account goes up 10%, you need to take back out $5,500. In this case, though, the IRS says that you WILL be required to pay income taxes, and possibly an "early withdrawal" 10% penalty on your extra $500 of earnings. That's not necessarily great news.

So in the end, you need to evaluate just how likely it is that your income will exceed the limits. The good news about contributing sooner is that you start getting tax-free growth sooner. The bad news about contributing sooner is that if you have to unwind your excess contribution and you have any growth, the growth must also be withdrawn and will be subject to income taxes and a potential penalty. That's a risk you'll have to evaluate for yourself, based on your own confidence about whether you'll come in within the income limits or not.

In any event, though, be certain to withdraw that excess contribution somehow if your income really does go over the limits. Otherwise, the IRS will assess a 6% "excess contributions penalty" against the ENTIRE amount of the contribution, and the penalty can continue to apply each and every year until you correct the contribution or it turns out that you were otherwise able to contribute the amount for that future year.

So in the end, I think that the early contribution is a great idea for those who are confident that they will be under the income threshold. If you think there's some risk that you'll go over, you need to weigh for yourself how dangerous it is and whether you'd like to go for earlier tax-free growth at the risk of owing taxes and penalties on your growth instead. And either way, if it turns out you DO have an excess contribution, be certain to unwind it by the deadline so you don't face any further penalties!

I hope that helps a little!