actually the screen size is not all that bad. ppc will be easier than palm though sadly (I have a palm zodiac with 480x320 screen) since I believe IE is built into PPC and your program appears centered around IE
your interface is already pretty small just get rid of the borders on the bottom left and right and move the icons to the top row as part of the status bar. this should give you about 320x210 pixels to play with on the zodiac and other modern pals keep the icons on the left override the status bar areas and you will have 460x320 roughly to play with.
most of your content will easily fit on either of these screen it will just have more vertical scrolling.
you have a lot of wasted space in your program as is some making it more dense for the smaller screen would not be all that hard. that is why I want ways to increase content density so I can use the full 800x600 of my tablet computer.
on the palm you would have to make a way to display your data since there is no web browser integration on the palm like you have in windows.
this would be quite a popular program for palms and easy to impliment in theory ie from a user perspective.
those without internet - you could have a PC side app maybe even built right into weather watcher download and parse a file for the PDA and "sync" it when the user syncs there palm. any data synced for allintents and purposes really is good for at least 24 hours. just have weather watcher update the sync file each time it updates so every time they sync there palm they sync there weather.
also make it live as many users are also ONLINE with there palms I have bluetooth to link up with my sprint phone and get online anytime I want.
this would be a nic app for palms and PPC's and some of the newer PPC's have 640x480 screens !!