At the time of this writing there are 22 OEMs (Original Equipment Manufacturers) that make Pocket PC devices. Just about all of those devices run the Flash Player for Pocket PC. The Pocket PC operating system currently exists in two different flavors, Pocket PC and Pocket PC 2002. For the folks who have Compaq iPAQs (StrongARM processor and upgradeable ROM), they'll be able to upgrade to Pocket PC 2002. For the folks who have older Pocket PCs with MIPS and SH3 processors, they will not. This gets really confusing really quick, so here is an attempt to help clarify things.
Devices that have MIPS and SH3 use the Pocket PC operating system, and are not upgradeable to Pocket PC 2002. These include older versions of the HP Jornada 540, 545, 547, also the Casio E-115, 125, and EM-500 that are all MIPS based. That means if you want to run Flash for the Pocket PC on those devices you'll need to use the Flash Player 4 for Pocket PC. Many folks out there will never need to use an older device or use an older version of Flash for the Pocket PC, but here's something to consider. On eBay, at the time this was written, you could purchase a Casio E-115 for $40. Using Flash for Pocket PC you can create a high-end Picture viewer, an MP3 player, and other standalone kiosk-like devices.
The Flash Player 5 for Pocket PC will run on all Pocket PC 2002 devices.
If you've used or read about Pocket PC devices, you'll notice that they all have a consistent set of specifications. Microsoft wanted to be sure all the devices have similar specs for ease of development, performance, and quality user experience.
It's a developer's and designer's dream to actually know what the performance is going to be on the user's device; in the desktop world it's often a numbers game where some people will simply not have the best experience.