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.
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).
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).