For 2010, the tax law has eliminated the marriage penalty meaning that you will not pay more tax if you are married than if you file Single. This may not be the case going forward. Congress has not yet decided on next years tax rates.
Regarding itemized deductions, if you file single, you get the 5700 standard, and your girlfriend itemizes, which must be somewhat more than 5700. You are right in that if you are married, you can either take the 11,400 standard or itemize. If her deductions are less than this you will be getting less in deductions overall being married.
Hope this is helpful,
You are correct. Keep in mind that you may have deductions to add to her 13,600 if you are married, like state income taxes and charitable contributions.