I am a UK citizen currently in the US on an H1B visa (ie, not a Green
Card holder) working for a US company.  I've been here for several
years and file US taxes annually.  I do not file UK taxes (not sure
whether or not I should, now that I think of it).  I am a programmer
(software engineer, software developer, whatever you want to call it).

I am thinking about moving to Canada (specifically Vancouver BC) but
continuing to work for my US company.  This might be permanent, or it
might be only for a year.  Leaving aside immigration issues except as
they might relate to taxes, what tax returns would I have to file,
what paperwork would my company have to do, and what (if any)
withholdings would I need to make or allow for?  What sort of filings
would these be (employee, independent contractor, individual
corporation)?  Approximately what sort of percentage of my income
would the taxes represent assuming $80k (US dollars) salary?  What
percentage at $100k and $120k?

Is there any difference in taxes whether I am a temporary or permanent
immigrant to Canada?

Thank you!