In my experience the Access applications I've seen are linked up to SQL Server or the native environment. Only in banks (especially Investment Banks) I've seen them linked to Oracle DBs.
SQL Server, Mysql, Oracle, db2, you name em, seen a lot of different databases over time.
Still love my first love though, Oracle course that is a lot to do with my knowing my way around it a lot better than I do others... SQL isnt that big an issue most of the time unless you get into analytics and such high end things. The functions like format, to_char, etc are radicaly unique to each environment which can make it a nightmare to switch from one to another.