Hi,
It seems they do not open in Leopard iChat 4 either - if opened as files
I have chats dated back to 2004 with the .chat ending.
Trying to open them directly says I can't run this App on this computer (Presumably it is trying to launch an earlier version of iChat which I have on another Boot drive)
If I launch iChat and drag the File to the DOCK Icon then the file will open.
I am not sure Logorrhea will work in Lion.
The Last update to it was in 2006 when it was written in Universal Binary (might work).
(it says 10.2 and Higher in the blurb on the page)
If I then Quit iChat and use the Right Click on the Chat file it says it is supposed to be opened with iChat 3.1.9 but offers me every version I have and I can open it with ichat 4.0.9 from there.
Does yours work this way ?
(The Info Panel just says iChat.app)

8:22 PM Tuesday; September 6, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously