Edit: sorry, missed part of your post.
Recovery is still working, so you can always flash the superuser package from the revolutionary how to thread if root really has gone. But this does seem very odd.
Is it just the superuser app that's missing, or is /system/bin/su missing too? You may not be able to see! Try installing superuser from the market and see whether it works. If not you'll need to re flash the superuser package (Also in the useful downloads thread, linked from the ATR Guide post).
Very strange that a nandroid might fail to restore a key element though - should be all or nothing...
Original post:
A2SD+ (ext partition) is not included in CM7, but is an optional add-on.
Either install Simple2Ext from the market or flash the a2sd script you'll find near the start of the CM7 thread (or DarkTremor a2sd, which you can find in the toolbox thread, linked in the downloads section of the All Things Root Guide sticky post).