Hi Karl,
Sure, there are several ways to do this. It's likely that you want to use a virtual IP address (VIPA), and run a routing daemon (OSPF or RIP) on the mainframe. With that in place you can use equal cost routing to take advantage of the multiple paths. The mainframe stack also allows you to do the same for outbound traffic. The usual recommendation is to use per-flow/session allocation rather than per-packet for traffic in both directions.
For more functionality, things like server health probing, deep packet inspection, various session allocation algorithims, geographic balancing, etc you can add the appropriate load balancing technology.
http://www.cisco.com/en/US/products/hw/contnetw/index.html
http://www.cisco.com/en/US/products/hw/modules/ps2706/ps780/index.html
There is also a Cisco load balancing solution designed primarily for the mainframe, as shown in this IBM Redbook.
http://publib-b.boulder.ibm.com/Redbooks.nsf/RedpieceAbstracts/sg246297.html
Rgds, Dan