you have to be invited by someone who already has gmail. i would invite you, but we can't share email addresses on here. have a look at http://www.gmailswap.com/, where you can offer to swap something (such as undying gratitude) for a gmail invite.
Thanks for that site, dafyd - am currently begging for an invite! I've wanted one for quite a several months now. I think someone may have just taken me up on my offer, too!
If anyone's *that* bothered then send me a message at <this username> at gmail dot com and I'll send a couple of invites out. I don't actually think it's as easy to use or wonderful as it might be