As it turns out, this contains two pieces of information: the actual serial of a unit of a given model, and the month of manufacture. The format is rather strange. The second letter and the first digit give the month of manufacture. These count like this: A0, A1, A2... A9, B0, B1, B2, etc.
A0 = March 1989. As you can see, there is no clean correspondence between the letters and years; you have to count it up. Or, the link above contains a table where you can look it up. C7 is the 27th month since March 1989, which amounts to August 1991. Since I bought the JD-800 in the summer of 1992, evidently the store had had it in stock for nearly a year before I purchased it.
The first letter and the last four digits give the actual serial, which is the count of units of that model manufactured. To figure this, you count the letter as A = 1, B = 2... J = 10, K = 11, and so forth, and you append the four digits to that number. So, a theoretical serial number of BD75191 would indicate the 25,191st unit of whatever model it was. However, looking at the JD-800 model number above, you can see that it begins with Z. What is Z? Well, Z = 0. So, my JD-800 is the 9,377th one manufactured. A thought about this: The JD-800 did not sell that well when it was first introduced, which explains why the store had it in stock for so long, judging from the manufacture date indicated in the serial number. And, mine must have been built near the end of the production run (actually, I wasn't aware that they had made as many as 9000 of them).
Looking at the rest of my Roland gear, none of them have a serial number beginning with any letter other than Z. I don't have a Fantom, a JV, or any of the more popular Boss pedals, and likely of the Roland gear I do have (other than the Juno-106, which was manufactured prior to Roland's adopting this system), none of them were made in quantities as large as 10,000 units. The lowest serial number of any Roland gear I have is ZR02527, which belongs to my Boss DD-20 delay (manufactured May 2003).
The system can go up to a unit number of 259,999 (which would be Y__9999). The page I linked to says that Roland has sold more Boss CH-1 chorus pedals than that, and that when it overflowed, they simply started over again with Z__0000. An interesting thing to note is that the date codes will overflow in November 2010, and judging from the above example, they will probably just start over again with that month becoming A0. The CH-1 chorus was in manufacture in 1989, so sometime in the future, it might be possible to find a pair of a very old and very new CH-1 with the exact same serial number.