It seems to me that my players (and I) are more likely to run away now than we used to be a few years ago. I think this is one of the few beneficial effects MMORPGs have had on P&P RPGs. Most in my group play or played EQ, and they have gotten very accustomed to seeing a fight go badly and having to bug out. In the old days they would have been more likely to view running as cowardly or dishonorable. Now they've learned better.
As a DM; it depends. I am more inclined to let the chips fall where they may than most, I suspect. (an overly evil tpk where the baddies sat down w/ popcorn to watch the cleric try and survive an Implode comes to mind...)
Although that did stem, in part, from the party's assumption that just because they were ethereal, they were undetectable.
In other words, if it is overwhelming because of poor tactics on the part of the players... too bad!
Personally, I'd run away. My players, however, never look for options much beyond kill or be killed. They will habitually fight to the death rather than turn tail, no matter HOW overpowered they are or obvious the escape route is.
As a player, I run away. As a bard, I fascinate the party member that's been taunting my bard-ness and run away.* As a DM, I like to think of such things as an opportunity for verisimilitude; I usually have some other NPC or NPC group somehow interfere, which may or may not be a good thing for the PCs, in the long run. At least it gives them an interruption in which to flee the battle...
* - Yeah, I know it's illegal, but I like the image...