The 110/112/114 iPaq Classic PDA is a great replacement for the 4150. It also has NAND flash memory, which you will never lose your data if the battery dies.. all new WM5 and WM6 devices have this feature.
Many PPC2003 programs will run on WM6. I have an HTC Touch running WM6 and some of my old programs work fine. When you install them, it gives you a warning message that this software was not designed to work on Windows Mobile 6, but ALL of mine have worked great!
I am seriously thinking of getting one of the new iPaq 100 Series as a backup device.... plus it has a much larger screen than most of the smartphones on the market (most have a 2.6" - 2.8" screen while the iPaq has one in the neighborhood of 3.5")
Enjoy!
