Well, I have taken apart a few Pocket PCs in my day, and I have been reading about this stuff for a long time, so I can knowledgeably comment on this.
I have thought about doing pretty hardcore-type mods before, but I haven't done much except overclock my friend's em500 and replace his touchscreen, my e115's touchscreen, and attempt to fix my iPAQ's touchscreen (a wire had gotten loose from it, still can fix it, though). But anyways...
There are plenty of things you can do and have been done or are pretty obviously possible. Different units have different capabilities, so I will use a couple examples. Let's take things that you can do with the e740 and things that have been done with the iPAQ and probably can be done with other PPCs. First, we will install a frickin' stereo audio-in jack (I don't know if that's easy to do with the Toshiba's audio chip, but the iPAQ's chip definitely supports stereo). Then, put a super-thin speaker in the back (the loud kind). Redo the buttons by putting better, e1xx/game-boy-style buttons (but ones that use the same electronics). Put a better quality sidelight in. Put more RAM in (PPCtechs or whatever has RAM upgrades for the iPAQs up to 256 MB, maybe you could work with them for drivers and stuff, maybe a joint venture!). Put more flashROM in (if you can find away to make it work like the new iPAQs). Put a higher-powered battery in instead of the one that's currently in it. There's an expansion-pack that adds USB and VGA-out, but that's basically just jacks, the actual interface is on the Toshiba and ATI chips. You should be able to fit that in in the bottom, instead of in the expansion-pack. Put a better 802.11b antenna in it, or the option of an external one. If it has bluetooth as well, do the same thing for that. Try to find a way to increase power on the IR-port. Provide an external serial-port adapter that's small (or even integrated). Wire the CF connection towards the back of the unit to allow for an optional and easily-accessible PC-Card adapter (There are drivers in the e740 for PCMCIA accessories, I checked.). Put a mini-switch in the side that gives you the option of turning up the speaker's gain (there's a mod to do this on the e1xx units, so I don't know if you can do this with the e740 as well). That's about all I can think of, unless it's possible to re-wire the connection to the ATI chip and increase the bandwidth from 8-bit to 16-bit (highly unlikely!). It may be possible that you can drive other LCD displays in the e740, if they use the same signal-types and power requirements. If the power-requirements are more, you can always mod it!
! My dad knows a lot about computers, especially on the electronics end... he used to service computer CPUs by replacing the transistors (obviously before they were chips)! He has been around awhile... and good at soldering. But whatever. I make it sound like I'm in elementary school... I guess those people are right when they say that once you move away from home, your parents seem to "magically" get smarter!
. That's all. Besides that, this device is almost perfect. But those things are a pretty big deal, but not so much in my opinion. Different people have different needs. And more money 
happy gaming guys!
