Time or Temperature

It's a bit of a mix for me. I definitely notice different flavors as a beer warms. I also notice, however, that I'm able to pick out subtler and more specific flavors/aromas if I've had one or two beers already and have a buzz. Don't know if it's maybe because my taste-buds are "warmed" up after a few, or maybe I'm just drunker than I think and am spouting out random flavors. Either way, I think both time and temp come into play when I'm enjoying a beer.

 

It's absolutely temperature.
As beer warms, malt flavor is emphasized. If it's not a malt-centric beer (pilsners, for one) fridge cold is perfect. If it's very malt-centric (e.g. English Pale Ales), I usually like 45~50 deg F. When I got into homebrewing, I realized I was always drinking Bass and others wrong -- they're usually served ice cold.

If possible - let the brewer tell you how to drink it!