Both sides seem to be going a bit overboard in this discussion... but that's just my opinion
On one hand, the Archos is definitely more compact than a UMPC and in my opinion the screen is good enough to view most websites without zooming, from my experience.
Pocketable/Foldable keyboards don't really take up much space, and could be used to enhance the experience with the Archos further, while still keeping the price quite a bit lower than with a UMPC. Would be nice to have the possibility of adding one via the Dock or the Dock Connector.
For someone that wants to run certain apps on a mobile computer but wants it compact enough a UMPC isn't a bad choice, although the prices are still quite high and the generated heat could still be less (the next gen from Intel could help there).
PDAs are quite another matter in my opinion, browsing is worse than on the Archos, given the resolution of the screen, writing text can be done by an additional keyboard, but combined its just not the same. Appwise more flexible, yes, but what the Archos can do it does mostly better than a PDA. (Videos, Music, Browser). PDFs i don't know how a PDA can handle them, on the Archos its slow, but thanks to the resolution of the screen text is nicely readable without adjustments.
I could go on in comparing but it's not that imortant to me... i got my Archos, maybe will get me another device for other needs, but i'm happy with it. UMPCs are out of the question for me currently. Since i already have a normal laptop, the price for a UMPC is much too high for me personally.