The blogosphere is jumping with the buzz that some users of the new Canon 5D Mark II have reported that photos taken with it can be "blemished with dark spots near areas with very bright highlights".
While some users have said that there is a simple fix for it, Canon appears to be aware of this report, and is looking into it...it's possibly a firmware issue.
I have two thoughts on this: Firstly, I never buy a just released new camera from its first batch, and always wait for the second, or even third, batch until all its kinks (if any) are resolved. Secondly, if this avers to be a real issue and Canon fumbles it, it'll have significant product credibility problems on its hands.
CNET's Crave Black Spots Afflict Canon's New SLR