Split PCI bus capability has become pretty common among the T-Class systems. The ability to split the PCI bus provides multiple I/O Domain support. With extra I/O Domains you can do all sorts of cool stuff like NIC fail-over and even direct card control.

If you've ever had trouble understanding (or in my case even just remembering) how the split bus configurations work, here's the site to check out: LDoms Community Cookbook, Section 1 - Hardware & Split PCI.

This page (and the whole site) is fantastic. Check out this image -- it makes understanding the bus allocations trivial:

I've definitely got to spend some more time on wikis.sun.com