I worked in Canada from Jan to April and then in the US from April to December on a TN Visa. My husband remained and worked in Canada.

Should I file my taxes jointly with my husband for my US tax return even though my husband lived in Canada and me in the States? If yes, I'm assuming he has to apply for a ITIN number?

Thanks!

Yes since you have completed 183 days on TN visa you have choice of filing joint return as residents. You will attach W7 of your spouse.
You must declare your world wide income for 2007 and then claim credit for the taxes paid in the foreign country.