A review of more than 200,000 4-digit PINs used on mobile phones revealed the following as the most common (in order):
1234 (used by more than 4% of the sample group)<
2580 (straight down the middle of the keypad)<
5683 (spells LOVE)
0852 (straight up the middle of the keypad)
The 10 most frequently used PINs represent more than 14% of the total sampled. Thus, with this distribution of PINs, you have a 1 in 7 chance of guessing the correct one in 10 tries.
Years are always popular when coming up with a 4-digit PIN (see number 10 above). So, birth year, graduation year, etc. would also be a good guess if these are known
Regardless, it's a very good idea to recommend people NOT use these particular PINs (at least the first 9 plus predictable years).
It does make me wonder about ATM PINs, though...