irt,
Learning any SQL rdbms, be it Oracle, Sql7, Informix, Mysql, Sybase, etc, will be of help since for the most part, the sql lanquage is universal. There will be minor syntax differences, data type differences, function differences, but again, these are minor.
As far as the hypothetical clandestine switch, you'd see more of a problem if you were a dba as opposed to a developer. For example, I have Oracle and SQL7 servers running at home on the same box for my development work. I have a test front-end in vb, where I've created identical back-ends in Oracle and SQL7 for some simple performance benchmarking. For much of this test app (granted, it's a very simple front-end) I can simply switch the connection string and do nothing else, and I'm running the different backend--totally transparent. Yet building the two db's had much more differences--I couldn't create a build script from Oracle and use it directly in Sql7.
jimmytheGeek,
OK, a Caprice

But one of the problems is the classic blessing/curse of ease of use, Wizards, etc. I've seen large sql7 db's that were created very quickly. However, simple dba rules are totally ignored, such as indexes and table data residing not only on the same disk, but in the same file! The performance is of course nowhere near what it could be, though when compared to something like Access, where some of these db's had a previous life, the developer/dba sees a big gain and figures the problem is solved. And yes, in Oracle the same mistakes can be and are made, but Oracle forces more thought to be put into critical things such as the basic physical db properties.
I guess it's my personal preference of control--a bit old fashioned I know--I'd rather have a bit more work in the details and gain the detail level control. Ie, I still rarely use Wizards of any type in Access, and I know a guy sitting next to me can have a form created faster than I if he's using the wizard, but in the long run, I believe my form is more stable and useable, and total dev. time is less, because of all the annoying 'defaults' that the wizard creates, which cause problems down the road or have to be re-done anyway. And I still order the stick-shift transmission too

--Jim