Knowing that Exchange Server 2003 supports active/passive clusters with more than two nodes, can we build clusters with nodes on differing types of hardware?
Exchange 2003 supports as many as eight nodes per cluster. In an extended active/passive cluster (aka an N+1 or hot-spare cluster), you can have multiple active servers with one or more passive servers that take over the workload if an active node fails. Microsoft uses such a configuration for its own mail servers: four beefy mailbox servers and a smaller server for failover support.
Although Exchange doesn't care what kind of hardware the nodes run on, when you're considering the supportability and reliability of the cluster, hardware makes a big difference. For any clustering configuration, don't buy systems that Microsoft doesn't list on its cluster Hardware Compatibility List (HCL). Some vendors do have dissimilar-hardware clusters listed on the HCL.