This document:
http://en.wikipedia.org/wiki/Generic_Ro ... apsulation
makes reference to an optional sequence number, and if i inspect the GRE packets on the Mikrotik, i notice that outgoing packets have an incrementing sequence number referred to as an "Identifier" (if memory serves).

I'm using a PPTP VPN, and the problem im seeing is that this value in the GRE packets seems to be stripped (zeroed) on the other side, for either the client or server side.

So my questions are:
Is this normal behavior?
How would the other side know if the packets are in sequence or not?
Am I just misunderstanding this?
Is there a buffer on the recipient side to re-order the packets?

AFAIK, a sequence is optional, but has to be "signalled" in GRE-header:
S, Sequence Number present. 1 bit.
If set then the Sequence Number field is present and contains valid information.

Otherwise, sequencing is also done on the TCP/IP-layer, wrapping GRE. So within GRE, is seems to be redundant, usually. Unless highest data integrity required.