Unfortunately, it's just not practical for something like SL for a number of reasons.
1. Lack of a good keyboard. Even with voice, there are times when you will need to type, or people who cannot or will not use voice. Not only are there no good keyboards for consoles, but they are usually very awkward to use in the environment that you find around a console (couch, coffee table, etc.)
2. Lack of system requirements. The X360 is, as computers go, a little lacking. It only has 512M of RAM which has to be shared with the video card. The harddrive is slow as harddrives go, and not every X360 has one. Games can be pre-optimized for consoles because the content is static. You always know what you will see or hear in a particular location. That's not the case in SL. SL is completely dynamic. Everything has to be streamed to the computer and put into RAM or onto the harddrive. With only 512M of *shared* RAM, and potentially no harddrive, this would be impossible, or just *painfully* slow.
3. Xbox Live! support mandatory. Now I'm not sure what is entailed in making a game compatible with Live!'s network matching, but I'm pretty sure that it would not be able to handle a system such as SL. There wouldn't just be a few servers, but literally THOUSANDS that would need to Permissions/Trusts through the Live! network. And very likely, LL would have to integrate some sort of hand-shaking software/system with each one. ANd I dont' know what Microsoft's policies on Live! servers accepting connections from the gernal internet as well.
4. ESRB ratings. The content available through SL ranges from the incredibly cute to stuff that proves
Rule #34. The ESRB would *HAVE* to give SL an AO rating. That's equivalent to giving a movie an X rating. Not only would it mean that most stores would not carry it. But that any parental controls will lock out SL as a WHOLE, and not just the internal mature content.
Very likely, if LL were to make SL for X360, you would end up with something like the Teen Grid. But without build tools; everything would be pre-made, pre-approved, and pre-cached. If that is something you might like, then I would reccomend getting a PS3 and using Sony's "Home". But a SL that's so limited would not be SL, but just a 3D chat client, and that's not what we're here for.

If it were, there are many 3D chat clients that are much more stable than SL.