As of May 1, 2010, the Ontario Harmonized Sales Tax will generally apply to prepayments, but only to the extent that goods and services are provided on or after July 1, 2010.
Ontario HST won’t apply to apply goods and services as of May 1, 2010. It only applies to goods and services that straddle or are used after the July 1st implementation date.
Here are some handy HST transition examples:

If someone purchases landscaping services for May 1, 2010 – September 30, 2010, GST is charged for May and June and HST …

The Ontario Real Estate Asssociation (OREA), lead by their President Pauline Aunger, present their perspectives regarding the Ontario Harmonized Sales Tax (HST) to the Standing Committee on Finance and Economic Affairs.
Pauline explains how the Ontario HST will affect not only the direct purchase of a home, but also the various services associated with purchasing a home, including appraisals, insurance and professional mortgage services. She points out that a $600,000 home will cost an additional $12,000 in taxes. But the HST doesn’t just affect home buyers, it affects current homeowners as …

We have received a lot of emails asking for a comprehensive list of HST exemptions in Ontario, so here it is. The following is from the Ontario Ministry of Revenue website.