I have and use my iPhone as pretty much as my main computer these days. So having DevonThink on my iPhone is really critical for me. I believe you will also have more and more people using the desktop client if you release a iPhone/iPod touch version. I personally did not use OmniFocus until the iPhone version shipped out and I used it for a day then bought the license for it! So I think a similar effect would happen in this case!!
I really do hope a mobile devonthink is in the works because it truly is critical for me and I believe many others - and also you will definitely get NEW USERS!