The busy bees at Wordpress have just released version 2.9.1 of Wordpress. This point release fixes a number of minor bugs that were seen as a result of the 2.9 release. A full list of fixed bugs can be viewed here
I have just upgraded all the blogs that I manage and all went to plan with no issues seen, clearly you need to make sure that you backup your database etc etc before attempting your upgrade. You can upgrade your wordpress installation manually by downloading the new release from this location or from within your dashboard using the auto upgrade function.