I don't ever remember being charged a cover charge for places in CityWalk, yet on Universals website, they are selling an "unlimited 1 night club to club pass" for $9.95. Huh?
Most of the clubs charge a cover after 8 or 9 at night, and it's usually $5 per person. You can visit the CW Entertainment website to see which clubs charge how much and what time the cover charge starts:
If you are in a club before the cover starts (say you are in a certain club at 7:30 and the cover doesn't start till 8), you can stay in that club for as long as you like without paying a cover. It's not like they go around table to table and collect the cover from people who are already there.
If you plan on club hopping, a CW party pass is a good idea. You can get them for $6 a piece (tax included) from Ticketmania.