Somewhat.
It's possible to write a webpage that looks almost as a native app for iPad (including multi-touch) but I had to give up with using "high level" events and had to handle instead the touches
array explicitly to get a reasonable zoom/pan. The results are IMO quite good (people I've shown that vector graphics editor toy thought it was a native app).
For Android however things are a little trickier because on my Nexus one apparently there is no way to get anything close to full-screen (and for a phone losing the address bar space means losing a LOT of space) and also multitouch is disabled in the default browser :-(
Both problems (fullscreen and multitouch) are however solved for example in Opera and this is in my opinion sad because (may be) this means they don't WANT good web apps on the phone...
So technically it's possible to write a single html5/js program that runs in both desktop and phone, but this doesn't of course mean that the best UI for a desktop app is also the best for a phone.