>is this still valid?

That was a quote from the Router Requirement RFC (RFC1812); it is a
Proposed Standard.  RFC2400 lists it as "Elective", which is described
as:

      A system may or may not implement an elective protocol. The
      general notion is that if you are going to do something like this,
      you must do exactly this.

implying that if you are going to do something like being a router, you
must follow RFC1812.

>Whats the status of the incremental checksum RFC?

Well, section 4.2.2.5 of RFC1812 goes on to say

   A router MAY use incremental IP header checksum updating when the
   only change to the IP header is the time to live.

and RFC1624 describes how to do it.  It simply describes the correct
algorithm to incrementally update the checksum, and doesn't say
anything one way or the other about checking the checksum on the
router.

  Bill