you can always contact one of your friends on myspace and they can get it for you from your profile page.
If that doesn't work, at the bottom of the page at
http://myspace.com/
is a Contact Myspace link. Use that link, and select "Account" using
the pull-down menu (the one that asks what your query is about).
Explain your problem IN DETAIL. That you've forgotten your e-mail
address. Let them know that you have the name the account was started
with, date of birth, any other information they should need to verify
that the account is yours, but that you're not sure what e-mail address
you used to activate the account.
Hopefully they'll be able to help you.