A few reasons I can think of....

- Getting a good email address is part of it. People see Gmail doing to other webmails what the Google search engine did to Yahoo and Altavista. Then they look at their xyz209384wqx@hotmail.com email address, and realize they'd like to get in on the ground floor and have something a little easier to remember this time around.

- I am sure that there is also some component of it that is related to the feeling people get when there's a velvet rope up at a nightclub that they can't get past. If someone else has something that everyone says is cool, people find themselves wanting it, just to have it.

- Google as a company has such a powerful brand name right now amongst geeks. They do a great job of using this positive karma to generate grassroots support of their new technologies.

- Everything Google's done they've done pretty well. They can walk into an overcrowded product space with a rookie effort and basically blow away the competition. They do everything right from a PR standpoint, too

- Finally, Gmail is just another webmail, but as Dan said, it's very well done, even as a beta. The positive reviews in the press make people want to try it out as soon as they can.