Help with Update and Merge

Question

Hi, I using http://support.microsoft.com/?kbid=320301 as a guide to building a webservice with a collection of methods for requesting and updating data. The client will request some data, change it/add more rows, then send it back to be updated on the server. When the client adds a new row I would like the identity of the row to be negative, start at -1 and move down. However when I'm populating the dataset I have to use FillSchema (ds,SchemaType.Mapped,"Details") if I want the Merge method to work properly. This prevents my id's from starting at -1, infact it causes conflicts. Because if the last id in the database is 60, then new rows added by the client will start at 59. Is there anyway to resolve this? Or should I omit the FillSchema method and just remove rows with negative id when the merge is complete? As the client devices are handles, I would like to keep any extra steps to a minimum. Any help would be greatly appreciated, Mark