I've stayed in convents on my last 3 Rome visits, including February of this year (there was light rain on and off during the week, and a downpour the day I left for the airport). I just had a PM from an FTer last week who also did the same thing, so don't overlook that. Pretty simple accommodations, and the price was right (think I paid around 55 euro a night this year and that included continental breakfast).

Convent listings:
http://www.santasusanna.org/comingToRome/convents.html

I have quite a few Rome photos linked below, including the convent where I stayed this year. It was about a 15 min walk from St Peters.