I really can't understand people that think that the 17" MBP is TOO big.
Look, I'm not exactly a big guy. I am 5.4 feet tall (165 cm), about 149 pounds (68 Kg), and I have rather small hands. And, still, I have been lugging around a 17" MBP (first a pre unibody, now an unibody) for the last 4 years and a half and I have NEVER felt it too heavy. It is waaay lighter than the 15" Toshiba Laptop I had before, and I will NEVER EVER get anything smaller than a 17" MBP for my daily needs.
What are those you say?
About 40% Word processing/web browsing, and about 20% video editing, 20%photo retouch/processing and 20% gaming ( yes I know MBPs are not for gaming but still they fit perfectly my gaming needs - I dont care i f I have to turn down the settings to make the game playable, i rather have everything i need in one sole machine than having to buy a fugly and bulky gaming Alienware Laptop). And for these needs, I really feel anything smaller than a 17 inch is way too small. Just try working with the raws of a 22 Megpixel DSLR camera, you'll see the bigger screen real state pays. Also on video editing: having several takes running at the same time to mix and match the edit is so much better on a bigger screen. Web browsing / Word processing??? Yes my child: When I write an academic paper i have dozens of other papers open to use as a reference, and a bigger screen helps to be able to write a document while at the same having a reference open and visible ( instead of zoomed out to hell). Gaming is the only place where a 15" may be better in the cases where I have to turn down the game resolution to 1680x1050 ( like BF3)but every other game I can run them perfectly at 1920x1200 (Civilization V and Shogun 2, for example)
The only time EVER when I have felt the 17 big is when I have had to get work done on a plane (economic class) or an interstate bus. And still I was able to get it done ... and even if I would have had a 15", I doubt it would have made a that much of a difference.
In this sense, I do understand the need of an Ultrabook or a MBAir. If I'm gonna go to Starbucks and ONLY browse the web while writing emails, well then yes a MB Air would be better. But then again, what if I feel like working on Aperture? On working on that video I was editing? I need the processing power of a MBP.
And for those of you who say that the cases and/or backpacks that can fit a 17" MBP are too big or bulky, then you my friend are obviously looking at the wrong backpack/case. This is what I have been using to carry around my 17 incher for the last 3 years and a half ( and looking good in the process

):
http://skattertech.com/2009/12/built-ny-laptop-backpack-review/
So in conclusion, If the choice was between a 15" and a 17" then for me its a no brainer : 17" all the way!!! ( even for a small guy like me). And if the choice was between a MB Air and a 13 " MBP, then for me the MB Air is the obvious choice.
Hope it helps