If you have an Eris or Hero, and rooted, its easy. If you have an incredible, you gotta wait a bit.
with the SDK installed...
cd c:\android-sdk-windows\tools\
adb remount
adb shell
rm /system/app/HtcTwitter.apk
I'm not 100% thats the file, so I would double check it. Its 3am and I'm about ready to pass out.