When you buy into a camera system, what matters most will be the lenses which you will be buying and using. The camera bodies can come and go, but the lenses you invested in will stay with you for a long time. Even if you get a small and light weight camera body, it will still become heavy when you mount a heavy lens.
For a start, the usual suspects recommended will be the Canon 550D and the Nikon D5000. They are good choices. But because they are full frame, you can expect big and heavy lenses, especially for the higher end range.
You may also consider the Sony A33 or A55. They are pretty value for money, lightweight and features-packed too. But again, like Canon and Nikon, expect big lenses.
Alternatively, you can consider the Four-Thirds (4/3) system by Olympus. They are much lighter and smaller dSLRs. As they are not full frame, their system is lighter and smaller, but still packs a big punch for hobbyists and enthusiasts.
I think you have got more than enough information about the pros of owning the Canon and Nikon, so I think I shall brief you a little about the lesser knowns.
For Canon, Nikon and Sony, the bodies do not have Image Stabilizer (IS), but can be compensated by their more expensive lenses that comes with IS. Unlike the Canon, Nikon and Sony, Olympus uses in-body IS. As their lenses are small and compact, they can afford to use in-body. Their Zuiko Digital Tatsuno lenses range are complete, more affordable and are superb! You just need a few lenses to cover a wide range. That's a bonus and saves you $$$. Here, you can choose either the Olympus E-450 (w/o IS) or E-620.
As mentioned by other kind souls here in clubsnap, Pentax is a good choice too. Perhaps the Pentax K-r. But again, lenses could be limited.
But if you just want a compact with dSLR qualities kinda cameras, you can consider the Sony NEX-3 or NEX-5, the Samsung NX10, or the micro Four-Thirds (m4/3). Sony is coming up with more lenses for their NEX system and Samsung has limited lenses for you to choose from. So the best choice could be the m4/3. The m4/3 manufacturers (Panasonic, Olympus and Sigma) are focusing and investing most of their resources in the development of m4/3, so you can expect more toys to play with in the future.
You may want to check out
http://www.four-thirds.org/en/index.html for more info.
If you can afford, perhaps the Fujifilm FinePix X100 can be your new toy too. But I doubt you want that now.
In conclusion, below are my recommendations (for entry/amatuer level) from each brand/system...
dSLRs
Canon 550D
Nikon D5000
Sony A55
Olympus E-620
Pentax K-r
Mirrorless
Panasonic GH2 or GF1
Olympus Pen PL-1 or P2
Sony NEX-5
Samsung NX10
Perhaps a food for thought... So far, I have recommended cameras to many of my family and friends. They have opted for the Canon 500D, Canon 550D, Nikon D5000, Sony A-550, Panasonic GF1, the Olympus Pen PL-1 and P-1. So far, with regards to weight issues, only my Canon friends are complaining about the weight weighing them down, especially during their travel trips. The m4/3 users have no complaints about weight and they have their cameras with them almost all the time.
All in all, remember that the camera is just a tool for the photographer... The choice is yours
