In migrating a site collection (not the portal root) of 40GB, the reverts never worked. The reverts timed out and it's this 'hang in the middle situation' where you still see the site collection listed in MOSS 2007 central administration but you cannot see its associating information (such as which content database it resides in). Of course, you also cannot see the site collection among the ones ready to be migrated in the "Select site collection to migrate" page.

This happened in one of the trial migration runs. Rebuilding the dev environment would be too time consuming because it involves migrating the portal first to get back to the state prior to migrate the specific site collection.

 So......

This discovery was by accident:

Before doing all these, you should check the "Database names" page for migration status, all the target databases are listed as read only

1. Disassociate the PAIR database (which contains the migrated portal) from the sharepoint farm in CA --> Application Management --> Content Databases
2. Re-associate the PAIR database to the sharepoint farm (that cleans up the 'bad' site collection associated with the web application)
3. Goes back to the site collection upgrade status page in CA --> Operations
4. Click "continue to upgrade" link next to the URL for migration
5. Click "database names" inside the Actions menu area located on the bottom left hand corner of the page
6. You can enter a new target database name for the orginal content database (change from read only to textbox for editing)
7. Click "Save" and wait for the operation in progress page to finish (If you don't give it a new name for target content database, it will create a new database with guid attached to it)

When you go back to the page that shows a list of site collection to be migrated, select the new target content database in the dropdown box and you see that all the un-migrated site collections are there (including the one that stucks in the revert process!)

One thing to note:
By associating to a new target database, there is no more options to revert those migrated site collections in the old target content database. But in my case, the reverts never worked anyway.