As reported on the Sony Vegas forum (http://www.sonycreativesoftware.com/forums/ShowMessage.asp?MessageID=743081&Replies=20), there is an issue with AVC renders in 10.0b that may affect some users.
Sony is aware of it and has offered the following answer.

Thanks for the detailed bug reports. We are currently looking into the issue and have determined that the problem is limited to the Sony AVC plug-in.
We recommend that you stick with Vegas Pro 10.0a if this is a workflow you typically use.
Sorry for the inconvenience.

Fortunately a user named Deti came up with the following solution.

Copy the AVC codec directory from 10.0a to the 10.0b:

32bit:
c:\Program Files (x86)\Sony\Vegas Pro 10.0\FileIO Plug-Ins\mp4plug3

64bit:
c:\Program Files\Sony\Vegas Pro 10.0\FileIO Plug-Ins\mp4plug3

If you don't feel like doing an uninstall/reinstall, user Dennishh has generously made the necessary files available for download on his site (http://dennishelmar.com/wip/)

Thanks for the info Mike. Is it necessary to delete anything from the directories mentioned or is all that is needed to do is copy the files that Dennishh mas made available on his site.

Update: I just checked and I see all the files have the same names so they are just replaced with the newer files.

Thanks for the info. This was helpful as I had some really interesting renders in 10b. The images where digitally smeared so they looked almost like oil paintings, but not what I was looking for!