Piper,
I just want to say that I think it is great idea to change units... mostly because I just did the same myself

...lol
I just found out that I was accepted to UAB for this fall, and I have no doubt that the way I sold myself on this issue and letting them know I wanted to challenge myself helped make that happen.
I worked on a CVICU for 2 years and have now transfered to a Neuro/Trauma/SICU. I too was nervous because I had become comfortable in my CVICU position and did not want to feel out of my element. But I also know that if I am going to go to CRNA school that I needed a more well rounded experience and a more solid foundation. I really think because clinical experience is so independent and diverse in CRNA programs that we (students) need to be as prepared as possible for anything, gain knowledge on the many diverse patient populations, and be ready to step up when there are unknow challenges to face. It is a hard choice to make, I don't see many swans anymore, but I do have a better understanding of a whole lot more situations, and looking a head I am ready for so much more after only 2 months of trauma patients.
p.s. make sure your current boss has already wrote your recommendations though, it is no fun to ask a new boss for that when they barely know you