Ubuntu Server is becoming more and more popular. While the Red Hat "wait five years between releases" model works for some applications, it is actually not very good for people developing new web-based programs. There is a lot of appreciation for a server-style OS that updates more frequently (semi-annually in Ubuntu's case), and Ubuntu offers contracts (supposedly) comparable to what you would get with RHEL so you can still keep the CYA business guys happy (and get support, I guess, if you need it).