the_david, I have been upping the difficulty for the encounters, adding more enemies and often increasing the level or HD of what they are facing. We have just completed Rhest as well, so I think they are on track level wise. Did have a decent amount of random encounters along the way.
Caedes, I have discussed one on one with the wizard and the barbarian (my roommates) and they are both aware and open to these ideas that have been presented. I will have similar conversations with the others as well.
Hopefully all it takes is a little more awareness on the part of the less influential party members to up their game. As has been stated, the blaster wizard is not overly strong, it is mostly that the others are not using their abilities to their fullest which is creating a more significant power dynamic than might normally take place.
nedz, As far as experience goes, the lower level characters are getting more XP per encounter than the others so as long as they keep showing up they will balance out in a few weeks. I feel like this method encourages people to show up but doesnt keep them behind forever.
ksbsnowowl and Gabrosin, I will check out the template, I like the idea.
Edit: I like the spellwarped template, definitely will be throwing that in on some baddies!