-- Last Updated: Jan-21-13 6:00 PM EST --
I see answers that seem to be relating to one-piece versus two-piece undergarments and also to the dry suit itself, so I think different people are interpreting that part of the question two different ways. I can't imagine Celia recommending a two-piece drysuit in preference to a one-piece, so I'm sure she's talking about the undergarments, and Matt is definitely talking about the dry suit rather than what's under it.
Regarding your later question about putting the dry suit over "jeans and T-shirt", the posters who talk about wicking clothing are NOT recommending jeans and T-shirts. You'll be a lot happier wearing some kind of non-water-absorbing fabric like polypro or synthetic fleece. You can't beat wool either, though it's pricey. I usually wear very thick, baggy, polypro longjohns under a dry suit. Rarely have I paddled in such cold weather that I needed another layer (even at around 10 degrees), though if I were a kayaker and thinking I might be swimming for extended times I'd include more insulation than I do. If you go with one layer, thick and loose long johns are nicer than thin and skin-tight (and that's 99 percent of the long johns that are made). Long johns with thick fabric that fit loosely are unbelievably difficult to find, but once you try some (for any outdoor activity) you'll probably be hooked (you never end up with that irritating, can't-wait-to-get-these-things-off feeling as you get undressed at the end of the day).