Have you looked at onyak tech?  They used to have some kind portal signup module...don't know if it will meet your needs but it's worth a shot.

I wouldn't create a DNN page per-user.  It will quickly become a messy solution when you try to manage the modules on that page.

I'm pretty sure the smart thinker and active social modules use the username for the unique page per-user. 

As for using Url Rewriting : you can't achieve this with a generic rewriter, because the mapping of the /username to a userid requires a database lookup.  You basically need to handle it at the module level : which I'm pretty sure the available social modules do in their later releases.  If not, the smart thinker modules are now open source, so you can just mod the code to lookup on username instead of userid.  You'll need to cater for illegal url characters in the avilable usernames though - probably best to handle this by encoding.

To my knowledge there is no easy and good way to do this. This is why many people have been waiting on the social networking features that were (are?) suppose to be in DNN 5 when it was announced 18 months ago.

The only solution I can think of right now is to build the basic social networking in Xmod, since Xmod will allow you to use the user name like you want and will also encode/decode the url for you. Since such an Xmod solution works the same way (by passing querystring variables) as Smart Thinker you could then also use Smart Thinker modules on the page. Although you would have to research whether all the ST modules would work - not sure if the friends feature would work in this type of setup.

Greg