As a Canadian permanent resident you have to file a Canadian Return, unless you abandon the residence. It is too late for this year.
so for 2008, by tax treaty you pay taxes on income earned in the U.S. and file a U.S. tax return. Since you have been in the U.S. longer than 183 days you will be fling as a U.S. resident.
By Canadian law, if you live and work overseas with no intention of returning to Canada you do not have to file. However, if you intend to return, then you would have to file taxes also in Canada.
Normally, all canadian permanent residents and citizens are considered to be termporarly out of Canada, and there for must file a return on world wide income.
However, when you disestablish all contact with Canada and establish permanent residence in another country, in this case the U.S. you do not have to file.
Under the conditions you have stated, you would be considered a deemed non-resident and there for only subject to tax and filing requirments for Canadian sourced income: