2 member replica set running version 1.8.3 + arbiter running version 2.0.
Stop primary.

Problem: Secondary does not become primary until 3 minutes later (see log below).

No problem if all 1.8.3.
No problem if all 2.0.
No problem if members 2.0 and arbiter 1.8.3.

Secondary log (primary was 10.182.38.10):
Tue Aug 30 17:52:38 [replica set sync] replSet syncThread: 10278 dbclient error communicating with server: 10.182.38.10:27017
Tue Aug 30 17:52:38 [conn5] end connection 10.182.38.10:34232
Tue Aug 30 17:52:40 [ReplSetHealthPollTask] DBClientCursor::init call() failed
Tue Aug 30 17:52:40 [ReplSetHealthPollTask] replSet info 10.182.38.10:27017 is down (or slow to respond): DBClientBase::findOne: transport error: 10.182.38.10:27017 query:

{ replSetHeartbeat: "rsA", v: 2, pv: 1, checkEmpty: false, from: "10.182.38.9:27017" }

Tue Aug 30 17:52:40 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:52:45 [initandlisten] connection accepted from 207.239.86.46:51353 #7
Tue Aug 30 17:52:46 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:52:52 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:52:58 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:53:02 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
...
Tue Aug 30 17:55:28 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:55:32 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:55:34 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:55:40 [rs Manager] replSet not electing self, not all members up and we have been up less than 5 minutes
Tue Aug 30 17:55:46 [rs Manager] replSet info electSelf 1
Tue Aug 30 17:55:46 [rs Manager] replSet PRIMARY