I just got this service text message from Rogers (at 1:48PM PST, April 6 2010):

Rogers svc msg:Good news! On May 4/10 tethering will be included at no extra charge as part of ur data plan on eligible devices.More info @ rogers.com/tethering

Checking out the rogers.com/tethering page indicates that it will be free for data plans of 1GB and higher, for both the iPhone 3G, 3GS, and also Blackberry customers.

You don’t actually have to wait for May 4th for this to take effect.  Rogers’ website states:

Effective immediately, tethering will continue to be included at no additional charge for Rogers customers who subscribe to a data plan of 1 GB and above*. This new policy replaces the current tethering promotion, originally set to expire on May 3, 2010.