How to hide updates from market? (Given Titanium backup "detach from market" doesn't work)
Save a copy of the original .apk file and keep it backed up.

A bulletproof way to get cut off from Google Play's update mechanism for that app is to re-sign it with a different key, then reinstall the newly signed apk file. Google Play (maybe) will still offer updates** for that app, but installing them will eventually fail because of the different signature. ApkTool will do it for you:

me@local:/tmp$ java -jar apktool.jar decode --no-src FDroid.apk fdroid/
I: Copying raw classes.dex file...
I: Loading resource table...
I: Loaded.
I: Loading resource table from file: /home/me/apktool/framework/1.apk
I: Loaded.
I: Decoding file-resources...
I: Decoding values*/* XMLs...
I: Done.
I: Copying assets and libs...
me@local:/tmp$ java -jar apktool.jar build fdroid/ FDroid.new.apk
I: Copying classes.dex file...
I: Checking whether resources has changed...
I: Building resources...
I: Building apk file...
me@local:/tmp$ md5sum FDroid.apk FDroid.new.apk
123c6932114dbed1334611eb8f4ca397  FDroid.apk
059ba98c5c60433cb6799013f22f244f  FDroid.new.apk
me@local:/tmp$ diff -rq FDroid.apk FDroid.new.apk
Files FDroid.apk and FDroid.new.apk differ.


ZipSigner2 might also help you do it (just found it on Google Play).