I don't know that one exists, it's been about a year since I was in charge of ordering PDA's but at my last look each company had a fairly unique connector port on its PDA.
My recommendation, pick a specific company (HP or Palm) and focus on choosing several of their models that have the same connection port.
No such beast my friend. And I would be willing to bet a large sum of money that there will NEVER be one.
I'll not get on my soap box about corporate greed, but suffice it to say that there is entirely too much competition and greed intra- and inter-company in this world... Such that one company would NEVER want to support another's product, AND that each new version of a product that comes out has a new accessory base so that old users have to go buy all new parts AGAIN, because none of the old ones work with the new stuff.
I have also never seen a PDA "dock" that has a video output of any kind on it. There are some (very expensive) cards that SOME PDAs can handle that will give you a video output, but they are not made by the manufacturer(s) and are very pricey.
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Docking Station for PDA