Lay of the land
The worst thing in my mind is walking on to a client site "blind" knowing nothing about who they are, what they do or how the place works. Just like the sales guys trying to find out who's got the budgets so they can squeeze a little more out of them, you need to know who's got the knowledge, makes the "real" decisions, how the place really works and who understands the systems more than anyone else. You'll need to make these people your best friends for the remainder of the project.
If you're part of an external group and there's an already existing web team within the organization, you should meet with them ASAP. Firstly because they might have no idea what's happening, or worse - feel threatened by your presence. Secondly, these guys' insights and knowledge of the organization and systems could take months or years for you to discover. Thirdly, ask about what they would improve, where they feel the system is falling down, what's the big hurdles, why has it reached the point of bringing in external people, that kind of thing. Make them feel an important part of the process, after all you've got to hand over to this guys at the end of the project.
