[Doitsu]

IMHO, there's either a bug in the Kindle 3.4 firmware or in the current version of KindleGen (V2.5 build 0626-3a91e28).
So far I've mostly used KindleGen V1.1 build 99 to create mobi6 files, because my K3 didn't support KF8.
Now that it does, I started experimenting with KF8 and encountered the same problems that the OP had. I.e. Go to Beginning no longer works in the .mobi and in the KF8 parts.
The Text guide item in my simple test file worked just fine when I compiled the ePub with KindleGen 1.1, however, it no longer worked when I compiled it with current version of KindleGen.
The strange thing is that Go to Beginning works in KP but not on my Kindle 3.4. I'm wondering whether this is a KindleGen bug or a Kindle 3.4 firmware bug.

Can someone with a Kindle Fire please open my combo mobi test file and test whether selecting Go to Beginning displays the first chapter? If it does, this is a K 3.4 firmware bug.

EDIT: For good measure I also compiled my test epub with Calibre. The Go to Beginning guide item works fine in the .azw file generated by Calibre. It really looks more and more as if KindleGen is the culprit.