[@aaclayton Participant 14 years, 2 months ago]

Sorry, I should have included this info in my initial post.

I’m using the bbPress plugin version 2.1.1
My theme is custom built using Justin Tadlock’s Hybrid Core version 1.4.0.

I don’t believe there should be any pre-existing theme conflict with the /users/username permalink structure, but obviously there is something that I have overlooked. If I switch to TwentyEleven, I’m not getting a 404 on user pages, so it has to be something related to the Hybrid framework. Hopefully I can track it down.