Hi there -- I have had a portfolio on childrensillustrators.com for several years. It is relatively inexpensive (about $200/yr) and every time I think I should stop paying for it I get a little job from it that keeps me there. Most of the jobs have been small, but this year I got a picture book job (I Love Your Heart, by Tim McGraw, due out in January 2010, Thomas Nelson), which blew my mind! So yes, real publishers are really looking, at least in my experience.
I have been concerned that the odds that someone who is just browsing will find ME seem small (there are lots of people on the site). However, the search mechanisms are pretty good as long as you put tags on your images to identify style, subject, medium, etc. Also, I have heard that putting up new work regularly makes it more likely that your images will appear in some of the random flash rotations that they have on every page (not sure it's true).
All in all, I would recommend it. Good luck!
http://www.abigailmarble.com