They've now made is possible to do "over the air" distribution, whereby you put up your .ipa file on your website and email your users a link. They can then go to this link on their iPhone or iPad and it will install from your site.

There's instructions on how to do this at: http://nachbaur.com/blog/building-ios-apps-for-over-the-air-...

Although it saves you from having to email large files, you do still need to collect your users' UDIDs and add them to your provisioning profile (embedded in the app).