By LV12-14 I had a corrosive rocket launcher, though in general corrosive weapons are pretty rare.
In terms of frequency (most to least) I've found it to be:
Fire > Explosive > Electric > Corrosive > Slag
I've got a slag sniper rifle and it's absolutely lethal, as it's combined with +120% Critical Hit Damage. They're definitely worth looking out for.
I've got LV17, LV16 and LV10 characters, covering three different classes. I'm maintaining two different co-op games and a singleplayer run through. So far I've managed to rack up 25hrs in-game and I'll be putting in a massive session tomorrow.
I'm annoyed that SLI isn't working. I'm using the latest drivers recommended for Borderlands 2 and yet the second GPU just seems to be doing a bit of PhysX. Performance doesn't drop below 60fps in singleplayer but co-op is much more demanding. I'm hoping that updated drivers and a patch or two from Gearbox will smooth things out nicely but I really wish this shit was sorted before launch. A quick Google search shows it's a common issue and not just specific to my system. And DiRT Showdown has plenty of SLI issues at launch as well. Not really that impressed with nVidia drivers.