The top long-term capital gains tax rate will rise to 20% effective 1 Jan, 2011, unless Congress decides to do something about it before then. (Will they? Who knows!! There's been talk about it, but, well, it's Congress. They don't even know what they're going to do.)
Anyway. The rules about when you can sell stock are mostly concerned with when you can realize a capital loss: if you sell a stock at a loss and then re-buy it for tax purposes within 30 days, it's a wash sale and not eligible for a deduction. However, I don't believe this applies to any stocks once you realize a gain - once you've realized the gain and paid your tax for it, it's all yours, locked in at whatever rate. Your replacement stock will be subject to short-term capital gains for the next year afterwards, and you might need to be careful with identifying the holding period on different lots of your stock, but I don't believe there will be any particular trouble.
Please do not rely entirely on my advice and consult also with your tax preparer or lawyer. :) And the IRS documentation:
Special Addendum for Nov/Dec 2012! Spoiler alert! Congress did indeed act:
they extended the rates, but only temporarily, so now we're looking at
tax hikes starting in 2013 instead, only the new top rate++ will be something
like 23.8% on account of an extra 3.8% medicare tax on passive earnings (brought to you via Obamacare
legislation). But the year and the rates' specifics aside, same thing
still applies. And the Republican house and Democratic senate/President are still duking it out. Have fun.
++ 3.8% surtax applies to the lesser of (a) net investment income (b) income over $200,000 ($250k if married). 20% tax rate applies to people in the 15% income tax bracket for ordinary income or higher. Additional tax discounts for property held over 5 years may be available. Consult tax law and your favorite tax professional and prepare to be confused.