Also, I've just found in the app-telio-30005.8931-S4-f2-reimage.tgz file the component that killed the USB booting in the EFI (and telnet). There's a payload called BIOS-telio-sec.tgz which is very explicitly written to zap the EFI with a locked-down version and disable telnet. There's some commented-out code that even deletes the telnet binary, so it looks to me like Telio were very much of the mindset that nobody should get in.
We're lucky O2 didn't go this route, or we'd not be Joggling like this!
On a side note, it's worth being aware that there's nothing stopping OpenPeak locking down any Jogglers running the native OS using their update system should they wish to, although I can't for the life of me see why they'd bother. I also made a little discovery that might circumvent the security applied to the Telio OpenFrame 2 units, but I haven't confirmed this by delving into the full payload yet. That's for another day, when I have more time.
