We are doing one for 2009 right now. You can recharacterize a 2008 conversion but cannot do a conversion after 2008 ends. Reason we are doing it early is because with the market down, we are being optomistic that we are selling at a market low and rebuying in a tax free forever account. Time will tell.
BTW, if you have Vanguard funds this can be done with a phone call -- no paperwork (but does not apply if you are doing it in a VBS brokerage account).