We have requested a crib the last 4 trips home and have had no problems getting one. Here's how to help assure you get a crib if you request one: First, attach the request to your reservation. This is no guarantee, but it certainly helps. Secondly, try to check in earlier in the day. We always check in around 1:00 PM and, like I said before, haven't had a problem.
Also, be sure to pack your own crib sheets. WDW doesn't use fitted crib sheets. They just tuck a flat sheet under the crib mattress. Finally, kids don't care that they're institutional looking metal cribs, just the grown-ups!