SODA Noriyuki wrote:
> We cannot change the format of wtmp, because that breaks binary
> compatibility.
> There are statically linked third party programs which read and
> write wtmp file (e.g. X11 terminal emulators write the wtmp file).
> So, if the format is changed, such programs just stops to work.
> --

But we can change wtmpx, that is used now. Wtmp now is used for 
compatibility only, AFAIU.