Having bought my first guitar a month ago, I'm particularly interested in tips for beginners!
I generally agree with what has been said - I actually like to know some theory and am lucky that I have struggled (not very successfully) with piano for the last 5 years - I find it remarkable how much this helps with guitar.
I tried to find a teacher but there don't seem to be any in my area, so I bought two tutor books. They seem about the same as my piano tutor books - not a great deal of help other than general guidance.
My breakthrough has been finding the Charanga PC based tutor which I am finding an enormous help and really enjoyable too. The great thing is that it is multi-dimensional and works similar to a live tutor. It has recordings of all the tunes so you can play along with any part (melody, strum, fingerstyle etc). It has just enough theory to keep you up to speed.
Only criticism? - it may be a criticism of guitaring generally but why do you have to use tab when notation makes so much more sense!