I came across a blog post about this limitation. If you are using ADO/ADO.net, you should be able to set the Page Size property to 999 - which will in fact return ALL users instead of being limited to 1000 (if you set it at 1000 or above, you will get exactly that many and no more, setting it to less really means "I want them all, but only x at a time")

Obj.Properties.Item['Page Size'].Value := 950
where Obj is an ADO Command object (not sure what the .NET equivalent is)