Doh, unfortunately, I'm on the 12.3 train, and that OID does not exist. I
could have sworn that I saw some MRTG graphs awhile back where people were
monitoring how many prefixes they had and other sorts of things. Were they
scripting this somehow or pulling via SNMP?
Jay,
The basic BGP mibs are found in this file...
ftp://ftpeng.cisco.com/pub/mibs/v2/BGP4-MIB.my
From that you can deduce the OID's for polling the routers (even 12.3 train) and collect BGP info. Alas you can't find out the one value that Jared pointed out... the number of routes heard from a peer. 
If you add the file above to your mib's directory then you can do a...
snmpwalk -v 1 $ROUTER_IP $ROUTE_COMMUNITY bgp.bgpPeerTable.bgpPeerEntry
...but you will only get the following entries in your response...
bgpPeerIdentifier.A.B.C.D
bgpPeerState.A.B.C.D
bgpPeerAdminStatus.A.B.C.D
bgpPeerNegotiatedVersion.A.B.C.D
bgpPeerLocalAddr.A.B.C.D
bgpPeerLocalPort.A.B.C.D
bgpPeerRemoteAddr.A.B.C.D
bgpPeerRemotePort.A.B.C.D
bgpPeerRemoteAs.A.B.C.D
bgpPeerInUpdates.A.B.C.D
bgpPeerOutUpdates.A.B.C.D
bgpPeerInTotalMessages.A.B.C.D
bgpPeerOutTotalMessages.A.B.C.D
bgpPeerLastError.A.B.C.D
bgpPeerFsmEstablishedTransitions.A.B.C.D
bgpPeerFsmEstablishedTime.A.B.C.D
bgpPeerConnectRetryInterval.A.B.C.D
bgpPeerHoldTime.A.B.C.D
bgpPeerKeepAlive.A.B.C.D
bgpPeerHoldTimeConfigured.A.B.C.D
bgpPeerKeepAliveConfigured.A.B.C.D
bgpPeerMinASOriginationInterval.A.B.C.D
bgpPeerMinRouteAdvertisementInterval.A.B.C.D
bgpPeerInUpdateElapsedTime.A.B.C.D
...where A.B.C.D is the IP address of the peer.
You maybe better off doing a poll of the router via a command line and plotting the values after scraping the text somewhat!
Martin