Depends really on the usage. Here's a quickie summary of the focuses of each device:

N800/N810: basically Internet focued, developer-centric in terms of interface design and tweakability.

TX: PIM (personal information management) is the best of the three; some multimedia focus, not as great on the web browsing side, but suitable for some things. Nice battery life given its age in compairson to the other devices.

iPod Touch: music and web focused. Moreso of a consumer device. Kinda like a Palm TX 2.0 in a lot of respects. Between jailbreaking and the upcoming iPhone 2.0 firmware, there are ways to add 3rd party applications that are quite nice. Right now, its not very good for PIM functions, but that will totally change when the new firmware comes and the apps come with it (many Palm OS 3rd party application developers are porting their software to the iPhone/iPod Touch including PIM software).

I would tell the person to wait as long as possible, and then weigh it like this: iPod Touch over the N800/N810 with the TX as a last ditch backup if the other two really are too over the head for usage.