Yeah... if you use "false" at the end of the command as suggested earlier... it won't rescan.
Like this:
importprivkey 5xblahblahPrivKeyblahblah "" false
NOTE: the empty "" is a "label" that you can apply to the imported addresses (the label shows up on the "receiving addresses..." screen and in transaction history etc, but isn't necessary to populate it, hence why you can simply use "")
Then, on the last key that you want to import, you leave the end part out, for instance:
importprivkey 5xblahlastPrivKeyToImportblah
And then it will do a rescan and scan ALL your addresses. It can save quite a bit of time if you are importing lots of addresses.