There are several beers that blur the lines between ale and lager. Here's a few I'm aware of:
California Common is fermented at ale temperatures with a lager yeast, most often Wyeast 2112 California Lager. These are also called steam beers, though the provenance of the name is not clear.
Kölsch: fermented at low ale temp (60°) with an ale yeast and traditionally given a long, slow age at low temp, similar to lagering.
- Rogue's Dead Guy Ale claims a German maibock style, but the recipes I've seen for this beer use Rogue's PacMan yeast, ferment at 60°F, and have no lagering period indicated. I don't know how Dead Guy stacks against, say, Hofbräu Maibock, but using a top-fermenting yeast and eliminating lagering (cold aging) would seem to disqualify it from claiming a maibock-style, but maybe that's too rigid.
I'm not sure there needs to be a definite line here.