SK2′s development hasn’t stopped by any mean…
Quite the opposite actually: I just released a new version today.

I just installed WP21 today (I had purposely avoided it for reasons all too obvious now) in order to check for compatibility problems with SK2 and found none… BUT I noticed WP ran even slower than before (if at all possible).

The problem you are describing above (a question mark showing instead of a karma rating) means that the WP/SK2 stopped before completing (usually because the server couldn’t take the load)… I suspect the added bloat in WP 2.1 is what is causing both WP and SK2 to fail and give up on comments (resulting in the ‘?’ you are seeing in the admin panel).

This is really more of a WP problem than a SK2 problem (SK2′s load on the server had decreased over the versions, if anything).

Anyway, let me know if this update helps with your problem…