Campus total at Iowa is 120 with chapters ranging in size from 40-50 to 120+.
As exlurker stated, many of the chapters have been there for 90+ years. Alpha Epsilon Phi is the newest chapter having been colonized in 2003. They are all housed chapters except for AEPhi.
Recruitment is a formal structured process with 5 days of events (welcome back day is split into two days). The chapters there are just as likely to bid sophomores and sometimes juniors as they are freshmen, so don't let the fact you're transferring be a deterrant. Be aware that all chapters do have grade requirements.
Quota for 2004 was 29. Iowa uses the new NPC process, so that number may or may not fluctuate for 2005.
Here's the Iowa Greek Community website. The recruitment page for 2005 hasn't been updated yet, but at least it will give you more information about the chapters and how to contact the Greek Life office.
http://www.uiowa.edu/~greek/