Howdy!
OK, first of all, I'm almost _never_ serious.
We just scored our first Mac in several years (my business #2 had a few for our design staff). It's a new MBP 13" (4GB, 250GB HDD, 2.4 Ghz C2D). We're adding it to a fleet of Windows machines, and it's looking like it will work out without many issues. I bought this after pretty extensive consideration of the wife's next machine. She also an iPhone user, and the iTunes (combined with iLife) experience is much better under OSX. I also wanted to revisit the Mac since I've always had an interest in them and am also considering some possible iOS (iPhone/iPad) development.
1) Is the standard 13" model sufficient for an average user? If not, should I go to 15" or just upgrade the 13"?
The 13" is pretty stout. Unless you're dealing with incredibly CPU intensive tasks, I'd say it's more than enough for most folks. We'll be using it for office chores, web, email, some photo management, handling iTunes for the iPhone 4, and some light video work (mainly short, personal movies). I'll also be messing around with some development, in my non-existent spare time
If portability is high[er] on your list (you know, THE list), then the 13" is pretty hard to beat. The 15" has the perk of a higher resolution option, a faster CPU option and a faster GPU (probably most crucial for 3D work, video production, gaming, etc).
FWIW, even though you didn't mention it, I was sold on the MBP vs the MBA because of the faster CPU, better expandability, and backlit KB.
2) Is it the right time to buy a MBP? Are there any updates coming that I should wait for?
Right now is an excellent time to buy. If you keep a look out, you can score some outstanding deals. Ours was discounted $150 (via a rebate), no tax, free shipping for a total of $1049. Of course, there's some latency with the rebate, but I don't have any worries about that.
Like someone pointed out, if you wait, you're just giving up time you could be using an outstanding computer. Also consider that Apple machines seem to have incredible resale value (vs. Windows machines), so if in 6-8 months, there's an amazing replacement, and I can sell this at a ~$200 loss, that's not bad "rental" cost.