“I had a Sys Admin friend look at it and he said that the “sign in/register” link appeared to require a template that didn’t exist in the theme.”
That’s incorrect. It doesn’t work based on theme templates.
Did you define a register slug?
Actually, the BP registration process does use registration/register.php in the theme folder. Can you check and see if that file is there.
It will use the BP slugs for registration if the template files exist, otherwise the old wp-signup.php or wp-register.php will kick in.
I’m gonna blame my faulty memory here. 
Thanks for the feedback, we’re still chasing this:
Ron – The Register.php file is there.
All – When we change the theme to kubrick, or any WPMU theme for that matter, it works fine for sign in – but of course it shatters BuddyPress.
Gordon
Eureka!
The problem seems to be that we had left the old BP theme folder in the content folder. Deleting the folder eliminated the problem.
…. just in case there are other hacks like me out there….
Thanks again,
Gordon
Thanks for posting the solution, Gordon!
I knew I had forgotten to do something.
I’m using the bp-twentyten child theme that Adrea_r made. It’s great but it has this problem as well.
I was curious so, I filled out the registration page on my site and then I tried renaming the entire theme directory, and then clicked on the complete signup button. Got a blank page..no surprise there. But after changing the theme directory back to normal, i refreshed the browser and got the next screen. This is the only way i’ve been able to complete a registration.
I’m sure there is something simple that will fix this, but i’d love to hear your opinions. I have no ‘old bp-theme’ folder as this is a fresh wp3.0 install with Andrea_r’s twentyten child theme.
Ps. Big thanks to Andrea_r! I don’t know how many of Andrea’s posts and tips i’ve used (as well as the new bp child theme), I really appreciate it!
I just signed up to your site. Worked for me. even got to the “hey, lookit your nice avatar!” page.
One thing I noticed – there’s not bp admin bar, no links in the menu, no indication anywhere about any BP areas.
As a new user, I’d have no idea where to go next.
But, then I did get my activation email, as I requested a blog as well. So, then I do see the admin bar when logged in. *phew*. So far, no errors though.
