I don't REALLY mind it either way, as long as they're not to the extreme - super pale so the person looks sickly, or super tan, and all the person looks like is one bronze sheet with two eyes (Tara Reid and Lindsay Lohan as of late have been evidence of this)...
Personally, I think a lot of it has to do with hair colour - if you have light hair, I'd rather the person be paler, rather than tan. If you have dark hair, you can go tan, but still, not TOO tan (Nicky Hilton sometimes does this).
Right now, I've been trying to watch my skintone this year, (I'm mixed-Asian, so I tan pretty easily) because I'm going to get my hair coloured lighter than normal, so I don't want to become too tan.
I think some of the most beautiful women, though, have been combinations of dark hair, and pale skin - Andrea Corr (see picture), Angelina Jolie...etc.
On men, it doesn't REALLY matter, again, as long as they're not to the extremes. A little tan is good, but a nice healthy flush is nice to see as well. But I certainly don't mind seeing men with some little sun-related freckles on their shoulders - it's cute.