Microsoft activesync can be set to convert pwi files to txt files as you sync with your computer. Somewhat complicated to set up but works well:
First copy your notes to your sd card and remove it from the phone.
Next synchronise your phone with your computer but in tools/options (pull down menu) don't synchronise files yet!
once synchronisation is complete the tools/advanced tools/edit file conversion settings menu will be available (it is greyed out until after you sync you see)
in that change the setting to convert notes to txt files.
then change the tools/options to synchronise files and put in the SD card
then synchronise again and the files will convert.
Do it in the wrong order and you will have pwi files on your computer and it wont do the conversion. If you delete them off the computers mydocuments folder it will delete them off the phone! (been there done that!) so then you need to turn off the file syncronisation option before deleting them off the computer and then turn on the file sync option. It then finds the computer does not have those files and happilly converts them to txt while adding them to the computers folder on your desktop.
The nice thing about this method is it produces a complete set of your notes all in their original folder structure so you can drag the whole folder into your android sd card or whatever