Tom,
Just a guess, but if you have installed any Office component or update, or installed a new version of the MDAC (which could be installed with VB.Net, Visual Studio, or MANY other applications, or as a standalone update), that could have changed the order of the libraries in the registry. It's not a large leap from there to seeing how, when something is declared as just plain old recordset (without the qualifier DAO or ADODB), that it would just grab the first one it comes to and go with that.
You will see that effect if you key in:
Dim rs as
then hit the space bar and get a dropdown list of all of the different variable/object types. If you scroll down the list, you will probably see:
Recordset
Recordset
These are the ADODB and DAO recordsets, but you have no way of knowing which is which, or which you will get when you declare a Recordset unless you specify the type of Recordset in the declaration.
Hope that helps.
Tranman