This came from the technical support section of Toshiba. At cedia a contact of mine stated that the TL-51 would work in 1st gen players. Due to the fact Toshiba has made every feature available on backward compatible on all players I can not imagine anything would change. In hddvd's current state it would be foolish for them to release a TL-51 and require a 4th gen player to decode it. That statement is as real as real gets and I have no doubt my XA2 will play the 51gb hddvd's with a firmware upgrade.
Naturally it would be in everyone's interest if TL 51GB discs could ultimately play in existing hardware.
However, a private conversation between you and someone at CES, or someone posting snippets of an unverifiable email in a forum thread are not *documented* confirmation. They may satisify your own need for confirmation (certainly a direct conversation with a trusted source), but they aren't documented or verified for a larger audience.
When Toshiba makes an official statement or a third-party is able to verify interoperability with TL software on existing hardware let us all know.