Call of Duty: Black Ops will have dedicated servers, but you don't get to poke around in them. You can play on the free ones that'll come online at release, or you can rent some hermetically-sealed ones from GameServers for you friends, but you can't host your own. You can't mess with the gravity.
You can't ban that overpowered weapon. You can't get rid of spawn times.
You can't put the Unreal sound effects in. You can't have it track your regulars and display kill-to-death ratios after each match in a makeshift league. Until - or unless - the mod tools are released, we're not sure you'll be able to mod it at all.
Update: I was partly wrong. Partly! See below for details.
I'm sure GameServers.com are decent providers, of course, and it's great for them that they've got the exclusive rights to host CoD:BlOps servers, but it's not clear at this point what the benefit are for the consumer here. Can you see yourself renting one of these? I want my own personal bubble to run strange maps on, you know? Sometimes you need a bit of RTD , or to mess around on a surf server.
Update: IGN have done a video interview with Cesar Stastny, Treyarch's PC lead for Black Ops, about the capabilities of the dedicated servers you can rent from GameServers.com . To their credit, the server package comes with a generous swathe of variables you can tweak and things you can do - including things I was sure you wouldn't be able to in my opening paragraph, like restricting the weapons. You still can't code your own mods, though.
Thanks to Forte in the comments for the tip.