Manuel is correct but the phrasing isn't too clear to me.  If I may:

> Is it possible to build an MP kernel from the 1.6.1 src?  [ship]

No, an MP kernel from 1.6.1 is NOT possible.  Manual said, "You need
current to run i386 MP."

> [snip]  I tried
> attaching the GENERIC.MP conf file from -current to the 1.6.1 GENERIC,
> but got the error message "ioapic*: unknown device `ioapic'" when I ran
> "config GENERIC.MP".  I don't know if that device is required...

Manuel said, "Yes it is." which I take to mean, "Yes ioapic is required."