Outside's Gear Guide for waterproof tents has this:
Anyway, some things to look for in general include: Bathtub-style floor, meaning there are no seams across the floor; full-coverage fly that goes right down to the ground all the way around your tent; and excellent construction using good materials. Many outdoor stores have several tents set up on the show floorójust walk around them and give the fabric a rub between thumb and forefinger to get a quick idea of the different thickness levels. And, remember, lighter tents mean thinner materials (usually).
My shortlist, then, for really waterproof tents would include the Marmot Swallow ($359; http://www.marmot.com
), a heavy-duty two-person backpacking tent that'll have you covered for some ugly winter weather. The really beefy fly and bathtub floor will ensure you stay dry in just about any conditions. Weight is nearly nine pounds, but the Swallow is a serious tent. Another good one is the Sierra Designs Omega ($289; http://www.sierradesigns.com
). It's a little lighter than the Swallow at about seven pounds, and includes a full-coverage fly. The floor has a seam, but it's factory taped and I'd be surprised if it ever leaked. Otherwise, solid materials throughout. Finally, Mountain Hardwear's Hammerhead 2 ($245; http://www.mountainhardwear.com
) is similar in weight and features to the Swallow with the advantage of a one-piece floor.