I posted this info before. Nokia will usually have their own roadmap in mind for when they want to release a device, but they don't give out a date in the very likely event that they aren't able to meet that date. With the N97 as an example, they can announce a 1H09 time frame with a tentative date to release being somewhere between April and May. Rather than be forced to rush a device and get it out by that date, they can push it all the way back to the very end of their projected launch to fix unexpected problems or further refine the software.
It's a great idea, in theory, except that those of us who are familiar with launch devices know that even with all that time they used to finalize the devices, they still come out very rushed.
Personally? I'd love for them to release a device with a specific date with what they would consider "pre-production" firmware, and then consistently update the firmware. It'd be the same experience we see now, except we'd have a definitive date on when to expect the device.