It depends on what you're doing. At many companies they will put both cluster nodes into the same rack and just connect via a crossover cable. Of course, if you lose both PDUs on the rack then you lose both nodes too.
The good thing about Windows 2008 clustering is that it will use whatever network has visibility to all hosts for the heartbeat. So if you did use a dedicated NIC for for the cluster heartbeat and the NIC failed the cluster will begin communicating heartbeat data over the public network (and indeed, the public network is how heartbeats are used with geoclustering).
From a best practices standpoint, MS used to say that running the heartbeat over a teamed connection was unsupported with Server 2003. I'm assuming that this has changed in 2008, but even on 2003 it still worked. It was just that if you called support they wouldn't support it.
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CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCSE:Security 2003
MCITP:Enterprise Administrator