There is an unspecified flaw in WordPress that could result in unspecified problems if exploited. I know that is rather hazy but the post on WordPress’ Development Blog simply says this:
We have a pretty important release available for everyone, it includes an important security fix and it’s recommended that everyone upgrade.
That is good enough for me. For them to use phrases like “pretty important” and “important security fix” and then recommending everyone upgrade tells me it could be a big hole that is being plugged.
At any rate, they recommend everyone upgrading to version 2.0.6.