That happens a fair bit, I've investigated several times and there is no rhyme or reason. Sometimes a Gmail account will work, sometimes it won't, it's not as simple as all AOL customers get blocked or something.
The madddening thing is that some new sign-ups don't receive their account activation e-mail. I'm afraid all I can really sugggest is you try changing your registered e-mail address and then see if that one works.
Have you considered making your podcasts into additional youtube links? I have a youtube modifictaion for the forum, so we could then embed your links directly into your forum postings. Another idea is to embed Windows Media Player into your webpages directly, like this: http://www.bloodforthebaron.com/toys/00 ... index.html
Happy to help if I can,