This is truly a question for the WPMU forums.
But if you look at the wp-config.php file, there is a constant defined for the overall site blog:
define('BLOGID_CURRENT_SITE', '1');
You can change that to:
define('BLOGID_CURRENT_SITE', '2');
But, first make a copy of your original wp-config.php file just incase something goes wrong.
This is actually more tricky that it sounds, because BuddyPress by default stores most of its information in the blog ID = 1, and it will ignore most other constant settings. If you are installing BuddyPress for the first time, and change any of the BP_ROOT_BLOG or BLOGID_CURRENT_SITE or SITE_ID_CURRENT_SITE variables, BuddyPress still saves most of its information in blod ID = 1.
https://trac.buddypress.org/ticket/823 references this issue, and I’ve included a patch for the instances where this occurs.
lynnallan, I’m afraid there isn’t a core solution for what you want to do yet, but the patch in the trac ticket *should* better prepare BuddyPress for this type of installation.
My only other suggestion, would be to think of it as WHERE the data is in the database doesn’t REALLY matter, unless you’re trying to split server load up across multiple database servers. That being said, you can install your corporate theme on your root blog, and then install the bphome theme on another blog, and then use the WordPress page template ability to recreate the look and feel of a BuddyPress on a blog other than the root. This is kind-of how http://leadpress.com/ has their site setup, if you need an example.
John:
I have been having the same problem since moving my bp install from id1 to id30.
I tried all the ‘suggestions’ on the forums and patched the files, but bp still seems to save all files like avatars, photo album plugin pics etc in id1 instead of id30…
Looking at the ticket, it looks like it’s assigned to 1.0.4… is this the case?
If so I am guessing that it would be best for me to wait for 1.0.4 to come out instead of trying to hack it together to fix it?
It seems really strange to me that buddypress only works on the primary blog. What I don’t understand is why this would be the case. Buddypress is so far away from what I would call ‘production’ ready status, that it makes no sense to me to run it on my primary mu blog. Isn’t buddypress in it’s current state better to keep in the background, away from others until it’s more stable?
not ranting here, luv buddypress, I just can’t use it as primary but do want it running ‘in the background’ so I can keep adding features and testing etc…
btw
(gave up on trying to update to 2.8 series.. never did stop spitting out errors, so prob just going to wait for 1.0.4 anyhow.. thanks for your time answering emails on it)
Of course John is correct. I failed to notice that you were running BuddyPress hence my “This is truly a question for the WPMU forums” comment.
There is a constant set in bp-core.php (line 15) that allows you to set the blog ID on which BuddyPress will run. But as the trac ticket John referenced in his post indicates, setting it to anything other than “1” will currently cause issues.
Also, this ticket applies: https://trac.buddypress.org/ticket/775
You guys are the best. Thanks for the time in answering the question. For the time being I have chosen a non-technical solution for the problem. I used domain mapping to set up the corporate blog on mysite.com and am using mysite.org as blogid_1 and my buddypress set up.
lynnallan, you could simplify that kind of setup further by using the multi-site plugin for WPMU, which allows you to map multiple domains to specific WPMU blogs. This way you can still keep a single unified login between all platforms. 
