Sub-selects are in 4.1, which is still in alpha, unlike 4.0 which is stable. (Again, this highlights the need for fact-checking when talking to MySQL fans!) And 5.0 is, at this point, vapor-ware.

As for your big system, true, you can still do massive systems that rely on '60s technology, just like they did in the '60s. Smart money says you'll upgrade to Oracle when upgrades start to come in late because your team is ferreting out every last integrity check that's strewn throughout the application code becomes prohibitive.