Hey Brian,
Brick and mortar stores have spent millions researching and testing the shopping process. Do they put everything by size? Typically this only occurs when there are broken programs or clearance. The reason is that people usually have an article in mind that they are shopping for (i.e. dress pants). If they put in size 32 and get shorts then they are going to be unhappy. On the other hand if they put in dress pants, they can then search by brand, color, etc and when they finally discover that perfect pair, then they can select the size. On the other hand, people shopping for a bargain in the clearance section are more concerned about fit and price since they have no specific item in mind.
In the case of Gap, it may work better since they have a limited selection (jeans).

I have to disagree with Douglas - I spent the first 7 years of my career working for retail companies like Gap and Eddie Bauer and the last 8 years in online marketing. I think the online shopping environment is very different than offline, and I believe the merchandising factor and visual appeal is much more important in the offline environment. In the online world people want efficiency and don’t need to see or interact with things they aren’t interested in. I think shopping by size can be a huge plus, especially when dealing with inventory issues. If an online shop can show me only things they have in stock for immediate delivery in my size it will be that much easier for me to find what I’m looking for and purchase.