Hello g8rfan
Yes, US funds are just fine. If you prefer to change your money to Peso's you can do so in the lobby, or the Gift Shop next to the lobby. The conversion is
about $1US = $11 Peso's.
Now, speaking of US cash remember as of yesterday, Sept 14 2010, Mexico's new law comes into play. I know this is not a factor for most on here, as most do not pay for their stay via US cash. However, this new law simply does not allow the resort to exchange any amount over $1500 US /person/month into Peso's . That means you can not walk into the resort, and pay for your stay in US cash if it is over $3000 US. You can however pay for say $2900 of it in cash and the rest on your Credit card.
Hope this helps.
Have a great day.