Linda, if memory serves, wheelchairs should be ok in Nassau proper, for shopping & stuff, though maybe some narrow spots. Atlantis itself is probably reasonably accessible, but I am not sure about the rides there - most cabs seem to be standard cars & very small vans, definitely not wheelchair-friendly. I agree with the previous post on St. Thomas; fortunately the shops by the main cruise dock are very nice, and easy to get to. In fact, a fellow shopper there was in a wheelchair, when I was there in Sept. Prices didn't seem a lot higher than in town, either.
Hope you have a wonderful cruise!