If you are using CS5 or CS5.5 (the latter is strongly recommended for all things EPub), "Convert URL to hyperlinks" is a menu command in the Hyperlink palette:
Yes, it's possible to do using GREP search. I have a collection of scripts dealing with hyperlinks I gathered from forums, sites, etc.
If you're interested, write me to askoldich [at] yahoo [dot] com.
Check out also the Hyperlinks section on this page.
Thanks for the information. I did a little research on using GREP and I've since whipped together a script that'll do what I need.