I have recently upgraded to TeamCity 7.0 (build 21241). I am not sure if this issue was present in version 6.
As you can see in the screenshot above, the most recent build in this particular configuration (22.214.171.1247 from Mar 5) was successful. However, when I click the "Last successful build" link at the bottom of the configuration overview page, I am taken to build 1.0.941.12 from Jan 18. Also, when I setup a snapshot dependency on the "Last successful build" from this project, it uses 1.0.941.12. It seems that despite the fact that this build was successful, it is not considered the "last successful build". Why might that be the case? According to http://youtrack.jetbrains.com/issue/TW-15628, the fact that the build number is italicized means it contains "out of sequence changes". I don't understand how this build could have been marked as out of sequence, but that is not accurate; it is using the most recent sources (as you can tell from the last part of the build number, which is the SVN revision). I cannot find any documentation about how a build gets "out of sequence", or how the server treats these builds differently.
I am currently unable to use artifact dependencies targeting the "last successful build" due to this problem.