Yes, I did use Web 2.0 tools with 4th and 5th graders. We used blogs regularly, and I did an action research project with 5th graders and Web 2.0 tools as a master's "thesis" paper.
Here's a link to tips when using web 2.0 tools with elementary kids: http://www.joplinschools.org/people/kreynolds/toolbox.html
And here's a link to the blog we used: http://classblogmeister.com/blog.php?blogger_id=82875
I'd love to help in any way I can! I train eMINTS teachers for our district now, so if you need some collaborative partners, just let me know.