(My fault in not answering earlier -- been buried in things to do!)
As MD says, "user accounts" can easily reach 100K without pushing your scalability, it's the concurrent users that count. As part of our work for the NZVLE project (see http://eduforge.org/projects/nzvle ) we are running a Moodle cluster which hosts ~40K users / 11K courses and growing. There was a lot of scalability work done in the 1.4.x series related to our setup.
We are now looking into a cluster setup with up to 300K accounts, and high number of concurrent logins (>1K).
Moodle scales very well, and we are putting significant efforts in reaching better scalability.
In reply to Martín Langhoff
Re: Information on Moodle Scalability (100,000+ users)
by Visvanath Ratnaweera -
Hello Martin (L)
I've heard about your project. Could you recommend literature, prefarably books, for the topics involved, MySQL (or PostgreSQL) clustering and tuning?
I've heard about your project. Could you recommend literature, prefarably books, for the topics involved, MySQL (or PostgreSQL) clustering and tuning?
In reply to Visvanath Ratnaweera
Re: Information on Moodle Scalability (100,000+ users)
by Martín Langhoff -
Search this forum! 
I've posted a lot with good hints, links to important tuning guides, etc.