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Hmmm...thats odd. If you're the only one that knows MySQL and people would be more comfortable to use ACCESS, what makes you think they are going to be comfortable with PHP. I'm guessing that ASP is what they want. I don't subscribe to this site but just in case you do get into that delimna you might want to take a look at this:
http://www.aspnuke.com
As far as your original question. The support for Access, PostGreSQL, etc.. has not been written yet. The good news is that XOOPS is the only CMS I know that have design this properly. They abstracted the database calls to a middle-tier and you can find all the source for it in /html/class/database. All they need to do now is build it for all these other databases. I'm sure someone can do it for you. You might have to pay them though