Figured the etiquette board could use yet another story of the impact/impression thank you notes leave on guests and people who send you gifts. I've been sending out notes as we receive items, regardless of the fact that our wedding isn't for another 2.5 weeks. In fact, I'm working on a few tonight.
A couple my parents know are aware of our wedding, though they were not on the invitation list. (They are business acquaintances- I've never met them, they've never met me). The wife chose to get us some things off our registry, after asking my mom where we were registered. After ordering, she asked my mom (via email) to ask me to let her and her husband know if/when we got the items. My mother replied wryly that I in fact sent handwritten thank you notes and had been up to date with everything I'd received as of Feb. 14th (I'm actually writing the note for this couple tonight). She explained that I was big on notes, and had spent my childhood firmly believing that accepting a gift without a written note to the giver = mom breaking my fingers.
Mom received a single line email response: "Wow- you two raised Chipmunk right!"
Even if they never say it to your face, people judge when you do (or do not) send thank you notes.