My SQL server keeps having the same problem. There are several tables that have a field that is an int, an identity, and is also the primary key of the table. Periodically, and for no apparent reason, the identity value will drop back, sometimes one or two numbers, sometimes 100 or more. Either way, since this field is a Primary Key, I can no longer insert any data into the table as the identity field is trying to number itself the same as a record that already exists. I've found the only solution for me so far is to remove the PK constraint, insert dummy data into the table until the index reaches a unique value, delete the dummy data, and then recreate the PK constraint.
Any idea what might be happening to cause this? Does anyone have a utility to correct the problem periodically, or is there a way to find the next identity value of a table without actually inserting, so that I can write my own utility to check the tables periodically?