i need to create a GUI app that will run on a windows PC, Linux X11,
Windows CE PDA, and the Nokia N800.

It will be basically a dockable tool bar with some floating bitmaps. It's like the bottom bar in windows with the start button. Normally it's docked, meaning all the other windows are smaller than full screen and make room for the dock. But you can also make it auto-hide, in which case all the windows are full-screen and go to the bottom of the screen, but when you move your mouse over the bottom edge, then it pops up on top of the existing windows. This app I need will work the same way, it will have 3 modes: 1) free-floating popup, like a normal window. 2) docked toolbar that stays locked at the top, left or right of the screen and the desktop and all other windows are reduced to fit within the dock, and 3) auto hide where it's on the top, left, or right of the screen but hidden until you move your mouse over the edge and then it
appears.
can u tell me a gui development environment which can help me for this?

I would suggest using Java since your main concern seems to be portability. Java is a cross-platform object oriented language and provides several API's for GUI development. The swing package in particular will be most helpful to you if you decide to go with Java.

I agree entirely with darkagn there. With regards to swing there are lots of great tutorials out there that you can look at too:

http://java.sun.com/docs/books/tutorial/uiswing/