I've looked into this and found the following information at the Ontario Ministry of Revenue, at the page Are You HST Ready?:
Will I have to register to collect the
Businesses that are registered for GST
will be required to collect the HST.
To reduce the administrative burden
for small businesses, Ontario will
parallel the federal small supplier
threshold. In general, businesses with
sales under the threshold (those with
total taxable sales of $30,000 or less
in the last year or $50,000 or less in
the case of a public service body)
will not be required to register and
collect tax. [emphasis mine]
Small suppliers that choose not to
register will not be required to file
a tax return and will not be eligible
to claim input tax credits. If a small
supplier chooses to register, it will
be eligible to claim input tax credits
related to its taxable supplies when
it files its tax return.
Note: Your final payment of Ontario Retail Sales Tax (RST, a.k.a. PST) would be due July 23rd, 2010.
Then, considering your taxable sales are under $30,000 per year, it would be optional for you to register for HST. However, there may be a benefit to registering since you'd be able to claim input tax credits – that may work out in your favor.