This is an old chestnut, but it might be fun for the few who haven't run into it in a while.
Continue the sequence of letters below. All 26 letters of the English alphabet will go either above or below the line. Once you've completed the sequence, assign a value to each letter starting your count from the beginning of the line. If the number is ten or greater, only use the right hand digit ("remainder mod 10" to you math folks).
Examples: D has a value of 3 since it is the third letter in its group. E has a value of 2 since it is the second letter in its group. If a letter would be the 39th in its group (which is, of course, impossible), the value would be 9.
The cache is at North 37:33.WSN and West 122:18.XTQ. Checksum is 52. (After posting this, a perfectly valid , but unintended, solution was presented to me. I've known this puzzle for about 40 years, but this is the first time anyone has come up with this solution. Hence the checksum. Contact me if you are in doubt.) Bring your own pen.
Update 12/7/2010: The object to which the cache is attached has been moved to the side of the large building. The offset is about 20-30 feet but readily apparent when you get to the area. If the restaurant is to your right, you are facing in the correct direction.