Hi Ted,
Here's an update. Since SharePoint 2010 and Office Pro 2010 are now officially available via MSDN I decided to take the big step of completely trashing my test environment and rebuilding the whole thing from scratch. The first positive thing I noticed was that the strange issue I had had previously (the DAT only running if I cleared out the form cache in InfoPath) has disappeared. Yay!
However, when I got to the part of trying to save the SharePoint tab info in the DAT all my old problems resurfaced. Beating the urge to throw my laptop out of the window was very difficult at this point but I decided to think about how I'd built my test environment. Everything seemed pretty bog standard apart from one "minor" issue. When I installed DBXL I had inadvertantly been logged in as the local administrator rather than as a domain administrator. I didn't think this was particularly significant but since it was the only thing that was different about my test environment (all other programs had been installed using the domain administrator account) I decided to remove DBXL and reinstall it using the proper credentials. I then created a new form loaded it into the DAT, filled out the Database tab, before finally filling out the SharePoint tab. This time when I clicked Save I received a positive response. Double yay!
Of course I've got this far before, so I decided not to get too excited. I checked in SharePoint and found the library that DBXL had created. At this point I remembered that you had to have data connections defined in SharePoint in order for my form to work properly in the browser so I went back to InfoPath and created these. Finally I clicked New in the SharePoint library and saw my form successfully rendered in the browser. The form is pretty basic. It only requests 4 pieces of information - Username, Surname, Forename and Department. The first three fields are simple text entries but the Department field is a drop-down box which gets information from my database. I completed the form and satisfied myself that the drop-down box was indeed populated. Finally I hit submit and received a reply that my data had been successfully submitted. Triple yay!
The final check was to look at the database. Would the data really be there? I had been at this point so many times before that my expectations were virtually zero but when I checked I was greatly relieved to find out that my database had indeed been updated. Quadruple yay! DBXL is working for me!!!
Now of course I have to try and get it doing something meaningful to wow the boss and get him to spend the money on it. With only a month left before my test license runs out I'd better get a move on, plus of course this only works at the moment when I use my domain admins account. I'm sure I'll have a lot of fun (NOT!) trying to get this working for ordinary users. :-)
A few final observations to help other newbies:
Be careful if your testing involves the use of beta software (obvious point really).
Don't install DBXL using a local admin account as this seems to cause a few odd problems.
Be prepared to do a lot of reading - you'll need a reasonable knowledge of Win 2008 R2, SQL 2008, SharePoint 2010, InfoPath 2010 and IIS 7.0 even before you start thinking about DBXL. I was blissfully ignorant about this when I first asked myself the question "How can I write to my database from a browser when I have no programming experience?" The whole process has been a complete pain-in-the-butt but I feel strangely elated today. :-)
Thanks to the Qdabra guys for keeping me on track and answering my many "dumb" questions.