Here is what they posted on the Google voice blog:
Tuesday, August 18, 2009 | 5:24 PM
In addition to making free long distance calls within the continental US, you can now call Canada for free through Google Voice. We offered this feature in GrandCentral previously, and in an effort to provide as many useful features as we can for free, we’re happy to bring those free calls to Canada to Google Voice users as well.
You can initiate calls from your inbox online, from our new mobile apps, or by dialing your own Google Voice number from one of your phones and selecting option 2 to place a call.
Please note that Google Voice is only available in the US, and you’ll need a working US phone to setup a Google Voice account.