As of May 1, the HST will generally apply on pre-payments for products and services that are going to be provided or performed on or after July 1. The HST should not be charged for any goods received or services performed before July 1. Read Canada Revenue & Agency Notice 247 posted here.

For Participating DMF Members:

Under the Transition Rules (see above CRA guidelines for Transition Rules) between May 1st and June 30th:
• Where the hotel charges the consumer HST then no DMF applies
• Where the hotel charges the consumer GST plus PST, then DMF is charged and remitted to GTHA per the DMF contract.
Commencing with the implementation of the HST on July 1st, 2010, DMF will no longer be charged to consumers and no longer remitted to GTHA per Section 6 of the DMF Agreement.

Resources:
Preparing for the HST - What You Need to Know
KPMG slide deck on the HST transitional rules, input credits, tax and compliance changes and other topics related to the tax harmonization is available here.
Canada Revenue Agency 
Ontario's Budget hotline: 1 800 337-7222 or visit ontario.ca/taxchange for general information on introduction of the HST in Ontario and wind down of the RST.