Though it may take a few months, here is what I would put together if I had the chance. Including Bluetooth, IButtons, solar panels and light emitting polymer screens...
For links to other linux handhelds, try linuxdevices.com.
My ideal handheld is the size of an A4 pad of paper, so I have to hold it on my left forearm with the fingers of my left hand curled over the end. A4 gives me plenty of screen space for watching real TV, reading real books, writing real emails, browsing real web pages and doing some real showing off.
The front cover is a solar panel, but I can't decide if the cells should be on the inside or the outside to help charge it while I use it or while I'm not using it. Hard one that.
The screen is not heavy-breakable LCD but LEP (brief technical primer, more on Google) or perhaps Xerox Electronic Paper seemingly available under the name Gyricon, pictures here and slight details here.
The choice of processor doesn't bother me much; I'd like to think there are many versions available of my handheld by many manufacturers (to drive the price down) and so many processors will be available but let's pretend the first release will run on a Transmeta just to keep excitement running high.
60 GB or so should be plenty of disk space, 2.5" IDE to keep weight down.
Input via stylus or sticky finger of course, with support for Graffiti, as used on the Palm and many others, also Quickwriting as featured on Slashdot as well as regular handwriting recognition (take your pick) and other pluggable input modules with popup keyboard for those times when you just can't manage to input a tilde (~) or backtick (`) properly.
Connectivity will be provided via a multitide of USB ports (where real keyboards can be plugged), Bluetooth (useless link) in action (good link), wireless ethernet as well as perhaps as many as 4 PCMCIA slots for things that change a lot like GPRS adaptors &c, or radio and TV tuner cards. Yeah! Why not add some Compact Flash while I'm at it? And boring 100 base T ethernet.
In fact I'm going to use the mobile phone card, along with my sound system to make the whole thing into a mobile phone for voice, not just data access. Talking of phones, the built in web cam can be used for video conferencing with (for example) Gnophone.
Better stick some firewire ports on there, too, for good measure, along with a few IRDA ports pointing in a few different directions for those more subversive inter-classroom networks as well as controlling my grannies telly to show off. And talking to my old non-bluetooth mobile which I can't afford to upgrade cos I spent it all on my handheld.
It will have integrated Ibutton support for security and authentification, maybe even built into the BIOS.
What more do I need? Oh yes, an Operating System. Pick your own.
I shall be running Linux with Ximian Gnome because it looks cool (and Bill Gates was nearly right, eye candy counts for a lot if only not to distract you by means of ugliness). I will be running redhat because I find up2date (or redhat channels of RedCarpet) invaluable effort-free way to remove those exploits, and I will finally get round to playing with Rebol.
The first thing I will need to develop is some network scavenging software to grab internet connectivity where it can for syncing imap folders and news, updating "offline web pages" [yikes! MS concept there]. Code to hi-jack available SMTP relays (*cough*). Does this smell a bit like Jini or something like it? I'll need to register my changing location for Gnophone so callers can find me. Perhaps the first thing for company visitors in the future will be to checkin their Ideal Handheld to the company network.
I will load all my favourite books into it as well as the entire classical Mormon works, copies of conference talks Doctrines of Salvation, Journal of Discourses etc, along with the Bible, Book of Mormon, and all of Project Gutenberg.
What will you do with yours? Have I missed any gizmos out? Or gadgets even?"