AFAIK, Rocky is still working on a WP7-compatible version of CSLA. I don't think he's finished yet, though - at least he hasn't said he is. Nor do I know of any official WP7 drops, even in alpha form.
I don't know whether you'll be able to get WP7 to work with pre-4 versions of CSLA, but I don't know you can't either. From Rocky's posts, it sounds like WP7 is a somewhat-modified version of Silverlight 3, with the main issues he's had seeming to fall under back-porting code constructs in 4 that don't work in WP7. So maybe you could get it working with potentially less work than what he's been going through. But it's been a while since I read the posts, so I may be remembering wrong.
If nothing else, you should be able to look on SVN and see if he's made anything available. I'm sure any code that's out there is "use at your own risk", but it would at least give you some idea of where he's going.
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