If you are coming to Pittsburgh with a large vehicle (higher than 6'), the Mellon Park garage at the Omni will not be able to accommodate it.
Your options are
1. Come in something else (OK, this may not be an option for some folk)
2. Look at the other car parks in the area http://www.city.pittsburgh.pa.us/pghparkingauthority/html/attended_lots_and_garages.html
Parking is cheaper in the North Side area (around the football and baseball stadia and the casino) and there is a free light transit system, known as the T, direct to the hotel.
3. If you are not going to be needing your vehicle throughout the conference, we are happy to have them left at our home, 20 minutes from the center (not on public transport routes). We have had a dozen vehicles in our drive and the neighbouring farm has a field abutting our property which is used for local special events and has had nearly 50 cars on it. Free but you need to sort out sharing a ride to town.