On MSNBC on Feb 15, 2008 they were saying for US customers
"... No one with cable or satellite service will be affected, nor will anyone who gets stations over the air with a newer TV with a digital tuner. ..."

The full story was here:
web link

On April 11, 2008 the word changed to:
"... Whether or not cable companies like King's can pass on costs related to the transition to customers is a subject the industry and the FCC generally avoid.
The FCC's digital transition Web site says if a cable company goes all digital and requires customers to get a set-top box, "any costs related to it will be determined by the cable company." ..."

resulting is this
"...So cable customer Doris Spurk was surprised to learn that thanks to the transition, she would have to rent a converter box for $5.95 per month, per television set, plus pay for a $60 service call to install it. ..."

The full story is here
web link

Now the question is what will happen to cable customers in Canada?
So far we have what sounds like the Feb 2008 words for the USA:
From the Canadian Office of Consumer Affairs....
DIGITAL TELEVISION
CONSUMER NOTICE
After August 31, 2011, all analog televisions will require a converter box in order to receive Canadian over-the-air broadcasts using an antenna. At that time, all over-the-air broadcast signals in Canada will be in digital-only format. Canadians using satellite or cable services should see no change.
U.S. television stations broadcasting over-the-air into Canada will make this change sooner, in February, 2009.

link to the site
web link