Any domain TLD can rank well. Google reps have confirmed many times that there is no part of the algo that scores based on the domain's "extension."
However, if you are using the .mobi domain as intended - to create content that is optimized for mobile access - I think it's very unlikely that Google's regular results would rank those urls very well. It would not serve their general end users very well if they serve the low bandwith content that mobile users want.
If I were to guess, I think it might parallel the way Google handles country tlds. As another thread here is discussing, it's very hard to rank well with a .ie extension on google.com.
<added> There's another critical reason why a .mobi domain will have trouble ranking for general and competitive keyword - how can it ever get enough legitimate backlinks?