Well, yes and no. You can still re-use
your html/js/css you made within PhoneGap, mostly - but you'll
have to use each vendors wrapper to make the final install file.
For Windows Phone that means using Visual Studio, Android will
need to use some form of Android SDK and Eclipse, iOS will require
Xcode to make the wrapper and a Mac to upload the ipa.
You can use something like PhoneGap Build (
build.phonegap.com) to
handle some of that for you. Sometimes it works nicely, sometimes
not as nicely. Still the proverbial "write once deploy
everywhere" doesn't really work just yet. The closest I've seen
for using a primary code base for multiple platforms has been with
Flash Builder, but I'm not here to start a holy war; and of course
that does nothing for Windows Phone atm.