Yup same experience here with the W Barcelona. Blue Palace, Crete charged me immediately. I made the reservation for W Barcelona the same day but watched months go by with no charge. I emailed the hotel and they assured me the reservation was in their system. I was still nervous because I never knew when they were going to pull the money and I didn't want to go into overdraft (I used a no forex debit/Visa card).

Westin Florence and Sheraton Diana Majecstic Milan both emailed me saying there was an issue with the CC I used when making the pre-paid reservation, could I please fax them the info. Not all of us have access to international faxing capabilities so I emailed them all the info but they insisted on fax. I ended up paying at check-in for Westin and cancelled the Sheraton since nothing had been charged.

I don't like being locked into pre-paid rates but sometimes the savings are quite significant.