A couple of them had Bluetooth. The only 68k-based one with builtin Wi-Fi was the AlphaSmart Dana, a writer's keyboard.
You're very very unlikely going to want to type a truly secure (= long) password over and over and over, which you'd need to do in a situation where the browser's password manager is turned off, and/or a website disables password caching anyway.
The Palm m5xx series could solve this problem: it had full USB, and I once read of an Palm app (like any other) that bridged the m5xx's SD slot to behave like a block device over USB, ie it turned the Palm into a USB SD card reader. That means there's a raw USB SDK out there, and adding HID keyboard support wouldn't be too hard (no kernel driver development etc).
Getting passwords into the device would be nontrivial; Palm keyboards are proprietary to the series they were made for, with a few arbitrary connector updates thrown in for good measure (think iPhone docking connector saga). If the password is irritating enough to repeatedly type on a full keyboard, it would take you a good 5 minutes (and a punching bag, for afterwards) to get it into the PDA, Graffiti and custom keyboards taken into account.
I think it would work out though: if the only way to get data out of the device is to tap something on its screen, that should be enough of a brick wall to dissuade would-be attackers.
I guess I'm responding to this so enthusiastically because it's about Palm :P - I unfortunately never owned one of these awesome little things, but I'd love something of similar capabilities built using today's tech. With modern advances in power consumption, like MemoryLCD, micropower CPUs, short-range bluetooth, etc, the result would probably last literally weeks. It'd be enormous fun to hack on, too, and carve out a little niche for itself. :)
I wonder if I should Ask HN if this would be a good idea.