1. Eventually we'll just skin the forums to fit in the same layout as the rest of the site.
2. IPB is not free software/open source, so using it means licensing issues that I'd rather not get into. There's a freely downloadable version, but the sources are encoded, making it difficult to actually modify.
As for a new skin, that's also on the back burner. phpBB let's you completely customize the skins, since they're just templates files, so I'll eventually make one from scratch that fits the rest of the site. I plan on making a new layout for the site first though. In the meantime, Zeality and a number of forum goers have been trying their hands at things, which is where we get all of the current skins. If you want to take a crack at it, feel free.
However, making some snappy looking forums for a bunch of half-interested 12-year olds to come and talk about how cool Crono or Magus are isn't the goal of the site. Popularity is a poor measure for us to use in determining the quality of our forums, or the direction in which we should move.