Short answer: You can upgrade to W7 Ultimate. It works and you should do it.
I believe I had the same promo copy (techdays or something). Mine was a full version with a good faith mention of you only using it for one year. It won't ever bomb out on you, it was just a request by MS not to run it forever.
Upgrading to Windows 7 Ultimate works, I did it with my copy. I'd guess you've probably installed the copy of Vista you got on a machine that was previously running XP. As XP is eligible for an upgrade license/legal/moral issues should be nil (infact I'd argue upgrading to 7 is better than running Vista for more than a year .. and that's ignoring vista being inferior).
If your machine wasn't origingally XP (or even licenced) you buying the 7 upgrade still puts more money in their pocket than you continuing to run Vista or returning to your old os.