Recently SS announced that they are going to provide RSS feeds of individual portfolios. I tried and it works great. You got nice image previiews in a feed.

I tried to check if other agencies got similar feed and it does not seem to be very common
1. Fotolia seems to have it but there are no previews, just title and link to image. For some reason it cannot be imported to FriendFeed but it works with Google Reader.
2. IStock - useless. Feed points to main site not your portfolio
3. 123RF - same story as above. I cannot figure out how to have feed of my portfolio.

Main difference between web page and feed is ability to quick show what has changed recently. When you look at page you can visually tell that it was different than couple days ago. When you use feed reader you will see what exactly has changed.