it's not weathersealed, so am wondering what, in reality, this means.

Well, I've used mine several times at a very wet Castle Combe circuit and have never had a problem. I just make sure that if it is raining, I keep the body in a plastic bag (or hold it under a brolly).

I've also used it in some reasonably hot and humid conditions in Europe (39degrees and 98% humidity), as well as taking pictures in some very hot (tropical) greenhouses. Never had a problem in either situation...

it's not weathersealed, so am wondering what, in reality, this means.
The 5D is not as weather proof as series 1D. Check out his link on Photonet Canon EOS Forum: EOS 5D and lack of weatherproofing? Is it a real problem? (http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00J1ak).
If you need to protect camera against the elements check out
The Kata E-702 Elements Cover and Optech Weatherguard can be found at Warehouse Express (I have no connection, other than spending too much money with them) under Lenses Waterproof cases.

I often use my 5d in the local Masoala Hall (Zoo Zurich), which has rain forrest conditions i.e. > 65% humidity and arround 30 °C. I also go there even now during winter time and I have never had any problems. When it is raining, I just take extra care that it doesn't get too wet. Hope this helps, Daniel