The AOLserver Tcl API allows scripts to generate multiple threads if you want, so you can already do asynchronous queries if you need to, though it would be a bit laborious. Each thread can grab its own db handles and query away to its heart's content if you want.
I don't think the ACS does this at all with code that hits the database. Normally, query results are needed to return the page to the user, and there's not much the script can do but wait until the query's done before continuing its work. I suppose it might be useful for background or robot threads in some cases.
If you were to implement something like this, you'd want to make the Tcl API database independent. That's one of the great things about AOLserver, and there's no point in breaking it. Since we're thinking about supporting InterBase as well as Postgres with OpenACS, we'd be unlikely to make use of anything that wouldn't make directly to other database engines.
