Well, I did it. I upgraded to Mandriva 2008.1 completely from the repositories, using an FTP program (ncftp) and urpmi. I did the base system first, then perl, then gcc, then kde, then a whole bunch of other things. I am now done, and the system is 2008.1. I don't even have a 2008.1 dvd.
The advantage to doing it this way? Well, it killed some time

. Also, I got a chance to look over pretty much all of the packages in the system while I did this, and that was useful...I did a reasonable amount of cleaning up along the way. Further, I did this upgrade while the machine was up and running, and I was even doing some work while it went on.
I only had to reboot once, and that was to change the kernel. I was down then for about 15 minutes because I also had to recompile the NVIDIA driver to match the new kernel.
This upgrade took a long time doing it this way, but I got a chance to clean up the system a bit while I did it, AND it went relatively smoothly this way. There were some hiccups, but I got to deal with them as they came up rather than have them propagate through an entire install and leave me with a mess, and the system was running and in production for almost the entire time.
Do I recommend this procedure to everyone? No. But, given that I have never reloaded since Mandrake 7.2 and just keep upgrading, for me it was a useful way to find some really old stuff and some broken packages that were in the system and get rid of them. Also, the total time I spent out of service while I debugged the upgrade was nearly zero, while the usual upgrade has me working for awhile to make the system function properly again. Whenever I hit a problem, I solved the problem before proceeding with the upgrade; kept everything manageable and the system was never out of service.
Wall clock time on the upgrade this way was a bit less than two working days. But during that same time, I completed a release of a little project of my own that I had been working on, established a place for it on sourceforge, and uploaded it. So I developed and upgraded in parallel. It worked out OK. Also, when you allow for the time to download the dvd iso, then install, two working days isn't out of line - and during the install the machine would have been out of service for a few hours, and debugging if everything didn't go smoothly would have kept it out of service for probably a few more hours. So this was OK.