Although all the usual suspects have already replied, I would throw my $0.02 in.
The EF16-35 2.8L is indeed a very nice lens and on a 1.6 crop makes a superb walk-around lens (although the range is a little narrow IMHO). I have chosen to go a slightly less expensive rout on my setup and am so far thrilled with the results. On the wide side I use a 12-24 F4 Tokina lens. This is a great lens, very sharp, constaint appeture lens. The downsides are that it flares and has above average CA. The upside is that it is about $450 new and bulit like a tank. L quality build. Superb optics but not L quality. I mate this with a 28-70 F2.8L Canon mid-zoom lens, and a 70-200 F4L telephoto. I tossed in a 1.4X extender recently for a little longer reach on the telephoto and have been very pleased with the results. With the 3 lens setup I have right now, I am pretty well set for most situations. Based on what you have and not knowing your budget I would recomend the Tokina. There are really about 4 UWA lenses in the range you are looking for: Sigma 12-24, Tamron 11-18 (, canon 10-22 ef-s, and the tokina 12-24. Some have suggested other lenses but the ones they have suggested are not any wider than your current 18-55 kit lens.
There are tradeoffs on each of the lenses I have listed and none are perfect. The Sigma can be used on a FF body, while the Tamron and the canon can not. The tokina can but it vignettes before 17mm. (making it a 17-24 lens). The Canon is the most expensive of the bunch and is only for the 30d, 20d, and rebel bodies. Can be a neg if you think full frame sensors are in your future, but I don't have my crystal ball handy. The Tokina I think had the best of blend of all the features, good price, wide enough, good optics. Only tradeoff is the CA and the flare. I am not a fan of Tamron, their lenses feel cheap in my hands and hare high on the plastic factor. The glass is fine, but if I am going to hold it and spend money on something, I want to enjoy it.
UWA photog is pretty fun, but it all depends on what and how you shoot. I have my 28-70 on the camera 60% of the time, the 12-24 10-15% and the 70-200 the rest of the time. If you want an UWA, get one,... but from my experience I use it less than my other lenses.... for what I shoot.
Remeber also that the 2mm diff between the canon and the rest of the field (1mm vs the tammy) is actually a pretty big difference. The Tokina's wide end is 20% narrower than the 10mm wide end on the Canon. That is a lot.
One board member said that you don't have a high res camera and that you don't need L glass. I think that is false. I guess nobody really NEEDS L glass, but it sure does help. Color, contrast, sharpness, focus speed, resistance to flare, all affect image quality (IQ). My brother still uses the 300D digital rebel because there is really not that much difference between IQ on that body vs the RebelXT, or the 20D, or even the 30D. I have an XT and don't see the need for the 20/30D for my needs, and for all intensive purposes, the sensors on the 3 camears are basically the same. My main point is that L glass may not make you a better photog, but it will make what comes out of your camera all that much better. Pictures have more punch, more pop, are sharper, require less PP, and are generally just 2-3 notches better with good glass.
RSO34 has great advice, I got to him for advice on this stuff as well. I think Akira3D also is spot on, although for me a 16-35 $1300 lens is not in the cards.
Last thought, some people say that UWA photog is a waste (for lack of a better term). They say they can take multipule exposures with a normal lens, get the same results but also benefit from less distortion and higher resolution. There are some really good panno software programs out there that can stich together pictures from even hand held shots. I did not go this rout, but you may want to consider this as well since it is a viable option.
Good luck, sorry for the novel.
a few reviews:
http://www.luminous-landscape.com/reviews/lenses/Canon-10-22mm-test.shtml
http://www.photo.net/equipment/tokina/12-24-f4/
http://www.kenrockwell.com/tech/digital-wide-zooms/comparison.htm
