CeZeus, you can win a cultural victory in Civ4. You can easily flip all but the capital cities of your opponents in Civ4, if you want to bother. However, their caps won't ever flip. You can block them in (take all their useful land away), but you cannot flip a capital. So no conquest through pure culture flipping of cities.

A cultural victory in Civ is an easy thing to do though, as it's merely a matter of getting a couple of wonders and all religious academies into 3 cities. You'll hit legendary in all 3 cities before you can get a space victory, at moderate levels of play. But you no longer get a default culture win (as in Civ3). You need to plan for it. Easy to do, once you know the trick. I suspect they'll close that down or move the line as its too easy to do currently.

I'm going to have to agree with Cezeus here. I have played a lot of games of Civ IV and other than on the easiest levels, it's virtually impossible to conquer whole swaths using culture. It is very easy to resist cultural assimilation in Civ IV.

The main issue is that there is no starbase equivalent in Civ IV to extend your influence.  Cultural victory at anything but the easiest levels is very difficult if you have more than say 4 or so players as it's difficult to beat them all to the needed improvements.