There use to be a few cars that were not technically allowed to be upgraded into (Hummer H3, Cadillac CTS, if I remember correctly). However, I've had a few CTSs and others have had H3s, so I don't think there's much that's off-limits anymore (at least in the US & Canada). I even had a new Camaro recently when I booked mid-size.

If you call the Chairman's Desk or the location (of larger/airport lots) a day or two before your flight, they'll note your reservation and usually make it happen. If you're renting from a small location, I've been told to give them at least 3 days for specific requests because they will bring in a car for me (at least my 2 local stations have done it)!