A review of more than 200,000 4-digit PINs used on mobile phones revealed the following as the most common (in order):

  1. 1234 (used by more than 4% of the sample group)<
  2. 0000
  3. 2580 (straight down the middle of the keypad)<
  4. 1111
  5. 5555
  6. 5683 (spells LOVE)
  7. 0852 (straight up the middle of the keypad)
  8. 2222
  9. 1212
  10. 1998

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. [more]

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).