That's a good point about scalability Michael - it's an interesting point these days as DotNetNuke starts taking hold in mainstream sites (WildVoice is looking good!) and userbase's start growing. The Core modules seem to handle ok (dogfooded by us) - but I think a lot of custom modules would need to be checked if your userbase was over a certain limit and stress tested thoroughly.
I would guess that the majority of DNN Developers do NOT architect their modules for 10 000+ users (a small amount for a successful networking site) - I definitely did not for my earlier modules as they started out from personal requirements for a small websites for friends (<500 users). For example, and drop down list of users or display of users needs to use database custom sp's to return paged data etc. I am slowly working through my modules and addressing this issues (Smart-Thinker is pushing 5000 users and the Invite Guest screen is almost unusable as it lists 5000 users on one screen(it takes time ;). For a lot of modules this may not be a problem (e.g. the HTML module) but for anything dealing with Users you should confirm the max number of users it was designed to work with (or the Authors intentions).
The Smart-Thinker HomePage module was designed for a small site, and while it does not make 1 role per user it was never intended to be used on a huge site - it currently makes a page per user and unless some things have changed in the core this would bottleneck at some point (Michael, I remember you asked some pertinent questions on max tab limit). I am almost inclined to say that if you are going to have a site with more than '0000 users you'd want to pay extra and hire an architect and design your own modules (or at least confirm modules could be used under stress testing). Look how the core Forum module had to adapt to the load - it's getting their slowly but surely.
I guess it's like in any industry - you might buy a small inkjet printer for home printing, but if you wanted to sell magazines you'd invest in a heavy-duty printing machine.
On a similar thread - I'm trying to gauge if anyone else is interested in a scalable, generic UserPicker that returns a list of ID's to any calling app - in case anyone missed it, I started the thread
here....