I'd also say the tent. Look at the offerings from Tarptent.com. They are significantly roomier and lighter than the others mentioned here. You should be able to save about 2.5 lbs for a 2 person tent or 3 lbs. for a 1 person tent.

But do look at all the rest of your gear. One of those "guidelines" for a lighter pack is that your "Big 4" (pack, tent, sleeping bag, sleeping pad) should be approximately half of your "base weight" (which equals everything except food, water, fuel) and "everything else" should be the other half. While you're looking for a lighter tent, work on the "everything else."

Particularly look at your clothing. If, like many people, you're taking several changes of clothing, for example, you're taking too much (my clean clothing, except for socks, stays in my car to change to for the drive home). I take as much clothing as I need to stay warm and dry, wearing all the items at the same time, in the lowest temperatures I could possibly expect (definitely lower than the average low, more like 5*F above the record low for the time of year), plus an extra pair or two of socks. In practice, that means my base layer stays dry in my pack in the daytime and gets worn at night in my sleeping bag, and probably in the early morning until I'm ready to pack up. Very seldom is it so cold that I have to wear the base layer for active hiking in the daytime. I therefore have a clean, dry layer to wear at night.

One good idea is to put a tiny piece of masking tape on each item before a trip. When you use that item, remove the masking tape. After the trip, go through and pull out every item that still has the tape on--it's a candidate to leave home next time. Of course, do use common sense--just because it doesn't rain for one trip doesn't mean you leave the rain gear home next time!

There are several gear lists on this site. Take a look at the "27-pound, 7-day gear list" on the home page of this site, left hand column. Also take a look at Phat's sticky post at the beginning of the Backcountry Beginners section. It has a good link to his gear lists. Comparing these lists to yours will be a big help in prioritizing what you need and what you don't need.

The other exercise I recommend requires only a scale, preferably an electronic postage scale which you can get pretty cheap. Weigh each individual item and note its weight on a computer spreadsheet. Be sure to classify the items by "system" as done in the gear lists I linked to above. Just this exercise will identify several items for elimination or replacement, either because you can easily find other versions or because they duplicate items you already have. This exercise also gives you a good gear list with real (not manufacturer marketing department) weights to compare to other lists, a means to evaluate possible new purchases (although weight shouldn't be the only criterion) and a handy-dandy checklist to print out when packing for each trip.