As promised when I did the initial review of my new 17" AlBook, I flew on an airplane yesterday. Not only was it a plane, but I was in coach and it was a Canadair Regional Jet (about the smallest plane that is large enough to use a laptop on).
The word is good. I was able to get my AlBook out from under the seat in front of my (from my revised Brenthaven Powerbook Backpack), put it on the tray table, open it up, and make fine use of it for almost 40 minutes of the 53 minute flight.
Now, it wasn't a perfect test, because the seat next to me was unoccupied, and the person sitting in front of me didn't put their seat back. However, I went out of my way to make sure that it was a reasonable test by leaving the armrest down and not putting my own seat back. Based on this, I'm quite certain that is you needed to use the 17" AlBook on a CRJ-50, you would have no problems, at least if you are my size or smaller (not too hard) and if you keep your seat in the same position as the person in front of you.
Since the majority of the weight in the 17" is at the spine of the book, you can feel safe letting it extend out beyond the front of the tray table without worrying about it falling into your lap.
However if you are one of those people who is willing to put their technology at risk by sitting a drink next to your laptop on the tray table, you are going to be out of luck in most classes of service. I don't do that anyway, but the 17" is a bit wide for most of the tray tables, and thus won't be accommodating for a tasty beverage.