The registration process on Aardvark provides a modal dialog that uses some nice content previews, content filtering, and progressive disclosure to guide the new user through the form.
1. Clicking sign up button from an invitation opens modal dialog with basic information form. A preview area updates your info as you fill out the form.
2. The birthday input filters and corrects date format. City/State shows suggestions, although I think the first one in this example isn't a valid suggestion.
3. A conditional select is shown for neighborhoods matching my city.
4. Following entry of email/password, additional fields are disclosed in the dialog.
5. Confirmation dialog is displayed.