I ride in the deep Southeast. What you should wear in the summer depends on two things: humidity of your area, and whether or not you have a full fairing.
A number of respondents here indicate they wear mesh jackets, etc. Those work well in the summer if your machine has no fairing or a skimpy fairing, as air flows freely through the mesh. But if you have a fully faired machine (BMW RTs, Goldwings, or Spyder RTs) you get very little air flow, so mesh jackets and pants don't work as advertised.
If you ride in a humid part of the country, you have an additional consideration, in that artificial fibers like Cordura don't wick at all, so you incur a double whammy. also, I've found that soakable vests and bandannas don't really work well down here either; they don't evaporate well, and soon you're riding with a warm, wet, dead fish wrapped around you. Out in the Rocky Mountain west, where there's less humidity, I hear these work well. They might work well on an unfaired bike down here, but I have no experience there.
So, after years on fully faired bikes, I've found that cotton wicks best. So I wear Diamond Gusset Defender jeans, with Kevlar at the knees, butt, and hips. (Hint: Diamond Gusset usually has these on sale between Thanksgiving and Christmas). Over my torso, I wear a thick, absorbent short-sleeved cotton t-shirt under a Draggin' Jeans Kevlar mesh long-sleeved shirt. The kevlar mesh shirt is still not as cool as just a long-sleeved cotton shirt, but the trade-off is, I get abrasion protection.
So while I don't have full padding, I still have some measure of abrasion protection all around. The cotton against my body helps wick away perspiration -- better than UnderArmour-type stuff (I've tried it; IMO, doesn't work down here) (not on a fully faired bike anyways). If I encounter rain, I either tough it out or put on a rain suit.
Gloves: don't wear black leather, get natural (tan) leather. Black absorbs the sun's radiation. There's a noticeable difference with tan gloves, especially deerskin. I got some at Bass Pro. Also, if you can't find tan gloves, perforated black gloves are preferable to unperforated black gloves.
I wear full, zip-up riding boots and an Arai full-faced helmet. The visor is always cracked because the air is so thick.
And for those wondering, in cooler weather I have an Aerostich two-piece Roadcrafter suit with all the bells and whistles.