It really is impressive. I used Icewm years and years ago on a little Ipaq, and it _is_ very fast, but it's really far too minimal for most users I think. I love it on the NIT (and FYI the Infadel2 theme looks beautiful on the Nokia), but then I haven't run a full 'desktop environment' (k/gnome/etc) for years. The tablets, in my opinion, require a bit more friendliness than a real desktop though- you don't have the advantage of a real keyboard and mouse, so having extravagant features like desktop icons are really pretty nice Fluxbox is my preferred window manager on a desktop, but without actual mouse buttons it just isn't practical.
LXDE does seem like the best tradeoff between performance and function. I believe that openbox is marginally heavier than Icewm (though I could be mistaken, I haven't done any real testing between the two), but they really are modular; it wouldn't be too hard to 'downgrade' back to Icewm and still keep the other goodies from LXDE.