In one of my posts I told about a technique to use multiple drupals in one database using prefixes http://dominiquedecooman.com/blog/how-run-multiple-drupal-installations-...
There is nothing wrong with this technique IF you start off with that setup.
But if you have multiple sites and you ve ALLREADY developed your sites using multiple databases and you want them REUNITED in one database. You should be aware of the following issue:
Drupal 5 uses the sequences table to keep track of autoincrement of node ids user ids, etc...
