It's a common question that comes from many customer in these days: all are worried about the coming of Dynamics NAV 5.0 and the impact that it can give to their actual applications.
Is maybe the time to say something about this "myth"? 
As I've said many months ago, Microsoft has decided to "split" the Dynamics NAV 5 platform under two releases: 5.0 (it's available for testing in these days) and 5.1 (end of 2007).
Dynamics NAV 5.0 will introduce some new features (links to record, new C/AL functions, new business functions, improved integration with Office tools etc.) but it's not a platform revolution. Dynamics NAV 5.0 client is still based on the "old" 4.x technology, so no changes on the backend technology and the old C/SIDE client is still supported.
The real platform improvement will be in the next 5.1 release, so an application upgrade to the 5.0 release will be treated as usual. Don't be worried about the 5.0 coming and prepare your customers for the migration... 