Hi marsha7,
May I ask, are you considering the purchase of your first digital camera? It sounds like you're debating whether or not your entry into the digital world should be with a more compact camera or with a move straight to a DSLR. Forgive me if I misunderstood, but I had the same debate with myself a few months ago. I ended up getting a compact (Canon Powershot S2 IS) last June, since I already had a film SLR. Once I got the digital, though, I admit the film camera has been sitting in the closet! I went through over 6,000 shots in about 3 months, took a couple classes on-line, learned a TON, and mostly learned that I was being hindered from continuing to learn by not having a DSLR.
I have the 5D, and it's definitely larger and heavier than carrying around the little Canon Powershot. I have a relatively small backpack camera bag that I use, so it's not difficult to transport my equipment. However, I haven't tried walking around the zoo, for example, with the 5D around my neck. I can forsee that being somewhat uncomfortable, but for me, the trade-off is having equipment that exceeds my current photographic abilities. I wanted something that I could grow into. When I get frustrated, it's usually because I need to learn something, not necessarily because my equipment is preventing me from going any farther. The flexibility of a DSLR cannot be beat, IMO. However, your decision really does depend on how often you'll be carrying your camera with you, in what circumstances, and what types of photos you hope to produce.
I currently have the KM 28-75 f/2.8 on my 5D (my chosen walkabout lens), and when fully extended, the camera and lens is 8 1/4" long. (Not extended the duo is 7" long.) The camera body is about 5" wide. This combo weighs nearly 2 pounds 15 ounces (so almost 3 pounds). The lens itself weighs nearly 21 oz. I think the 28-200 you spoke of weighs around 17 oz (?) and measures just over 3" long (?), so this would be a more compact combo.
The advice given you about buying the A2 and then keeping it as a compact back-up camera if you should decide to get a DSLR later is good advice. On the other hand, I'm not sure how much budget factors into this, but if you end up like me, then spending a few hundred dollars now on a compact and then wishing you had a DSLR in a few months could get expensive! How far do you want to go with your photography? If you think the 5D would be all you'd ever need, then why not just go for it? I know...the weight and size issue. (By the way, the 7D is definitely larger than the 5D, so that is one reason I didn't consider it. The 5D fits very nicely into my hand, isn't too light to feel flimsy, and has nice ergonomics with the placement of the buttons and dials, at least for me it does!)
I think it comes down to what you're willing to compromise on - carrying around a larger and heavier system for increased flexibility and quality (these factors also depend on what lenses you choose), OR carrying around a more compact camera (which you might carry more often because of the size) but perhaps miss out on some of the flexibility (low light shooting, different lenses for different situations, etc...) and quality (less noise, sharper optics, etc...).
Good luck in your decision! Whichever camera you choose, just enjoy it, use it a lot, and share some pics!
Elaine