http://www.spam.co.nz/zinwell for the 620/630/640 Zinwells :-)
It used to be someplace else but I decided to move to this domain now and will be redirecting from previous location to new location in the future)
As I have said in the past I have asked Zinwell (Taiwan and Australia) and NeXT (before they went into receivership) MANY MANY MANY times for this software directly and no one ever replied to me.
I was told 1.2.2 was the latest firmware for the 640 a while ago but I have 1.2.4 as well but I don't know the differences between them. Also to note these are upgrades only form existing firmware because as far as I know the full files where never released.