If you include your spouse in any way on your tax return he/she must have a TIN, even if they are a nonresident alien. Use form W-7 to apply for one. Having a TIN doesn't change their immigration status or entitle them to any Social Security benefits etc. It is strictly used for identification purposes. Requiring either SSN or TIN number on tax forms for every individual claimed stopped a lot of tax fraud.
Quote from IRS 1040 instructions:
Nonresident Alien Spouse
If your spouse is a nonresident alien and
you file a joint or separate return, your
spouse must have either an SSN or an ITIN.
I suggest you take a look at Publication 519 on the IRS website. Bottom line is pretty much this, if you want to receive the tax benefits available to a married person then you have to do the same as other married couples and report income from both parties.
Of course, there are always other things you might be able to claim on your return to reduce your tax liability. For example, if you live and work overseas you might be able to claim foreign earned income exclusion or foreign tax credit.