Canadian immigrants from pakistan never required visa stamp while entering into US, because of Pakistan is being a british commonwealth country. So they require to present their landing paper & H1 approval at the border post. But in present climate, rules & policies are changing, and I heard and read news that that they are going to impose visa restriction on all or certain canadian landed immigrants of british commonwealth countries including Pakistan and implementing it may be from Dec 16.
So if you have visa sticker in your passport you dont need to worry about re-entering into US, to be on the safe side try to make a trip and return to US before expiry of your visa or before Dec 16. You may also be aware of special registration requirements for pakistanis... so when you will reenter US INS will probably be taking your fingerprints & photographs.
You should also call INS/INS-border post before leaving US for Canada to ask about any re-entry policy changes.
Please dont forget to post your detail experience here when you are done with your trip to Canada.
GoodLuck
