Last week i had to do some maintenance for a customer.
During this maintenance we had serious trouble with the bdc-definition.
We started out because the customer had serious performance issues. We decide to change 2 things, just for the test.
One of them was > recreate the Shared Service Provider
During installation someone installed the SSP on the default webapplication (port 80).
So now the SSP is part of the portal.
We decided this was not a good situation and started planning for a migration.
First, gather all current information about the SSP and check what will be lost and what not.
One big thing at this customers place is that we use a BDC that is vital for the working of the portal.
So we tested in a "test environment" and if we export the application definition file and import it into the new SSP , everything should continue to work. (we had some custom pages that rely on this bdc-file)
First issue : When ssp is configured to use with the default webapplication, the only way to add the default webapplication to the new SSP is to remove the original SSP. Hmm, i didn’t like that, because we might need some information afterwards. But we decided to continue and deleted the SSP (remember : removing the ssp from the default webapplication will NOT remove the webapplication, retaining the default portal)
As it seems everything was working, ……. until we checked the BDC .
After re-importing AND resetting the security> the application was not working !
Checking every website in the google universe , we never found a solution.
The bdc-file was imported correctly, we could set permission on it AND we could see all settings.
But when using the BDC-picker tool, our application was not visible. 
At this moment we were getting desperate, until we found somewhere a note of someone (i really don’t know where) that it could be search related.
And one thing we hadn’t done yet was: Start up search. Because of the impact we decided earlier that day to wait until the weekend. We decided to start the search and wait until the weekend passed.
AND GUESS WHAT : the bdc was working as intended.
what is the bottomline of this story : somehow BDC is relying on a working search environment. Some make sure you run a full crawl to the end, before starting to troubleshoot BDC-related problems.
In our situation: the search had never run before. (Search is part of the ssp)
Hope this will help others,
Kind regards,
Eric