>I'm using routines from the downloads area for Credit Card pre-validation (to verify bank codes and checksums) - Thank you Steven Ruhl - but are there similar rules for checking account numbers and routing codes for those who pay by check?

There is for the routing number
You sum up the first 8 numbers of 9 digit number by the following formula
1st Digit * 3
2nd Digit * 7
3rd Digit * 1
4th Digit * 3
5th Digit * 7
6th Digit * 1
7th Digit * 3
8th Digit * 7
Gives you a total then take the right most digit of the total, if it's not zero, minus that number from 10 (if it's 8 take 10 - 8) That number should equal the 9th digit in the routing. The route number has to be 9 and all digits.

As far as a checking account number I don't know if there is validation for that.