They came up with the rule that you can, as long as they don't look like the adsense unit and users don't get them confused...hard to do when blending eh?
Of course you can use all three of them together in a single page as long as they do not look identical.
I'm designing a new website this weekend, using Adsense and for the very first time Adbrite. Adsense is ok, but I'm anxious to know how Adbrite will react to a French language site