one day I noticed my registration sticker was out of date, then realized that I did send in the check and that it was cashed.
I went to the (California) dept. of motor vehicles and asked them why I never got them. they said 'that happens sometimes, here..." and they had me fill out a few forms stating that I never received my new ones. then they charged me about $15 for (more) new ones.
when I got home to put them on my car, I noticed that the 'sticky' bit was no good at all on the new sticker. so I went back again.
you guessed it, more forms and another 15 bucks !!!

on a side note, I have also been told by the DMV that the notice you (might) get in the mail is not guaranteed and it's up to you re-register you car with or without any notice.