It means exactly that, the numbers ran out.  Companies buy batches of ISBN's depending on their needs.  The first allotment of TSR ISBN's was only 100 numbers, now it's somewhere around 10,000. In order from oldest number set to newest it goes like this.

0-935696-*

0-88038-*

1-56076-*

0-7869-*



Sometimes a change in ISBN will signify a company change, but that isn't the case for the D&D products.

Plaag wrote:Did TSR put out products in numerical order for the last #'s of the ISBN, or did they skip around?

Typically when you decide to publish something you circulate the data (Title, stats, ISBN) well in advance. Sales/Marketing typically handles that, simply taking the next ISBN 'on the stack' for that series and assigning it to something. Stripped/sequential ISBNs are typically assigned to a specific group, e.g. novels, modules in sequence, etc.

Again typically, this all occurs before production even begins, before something is even written. Obviously production delays, marketing decisions to postpone items, etc. can thus result in a shuffling of ISBNs.