Another assumption on those supposedly home-brew switches is that they be designed around Broadcom's BCM8706's platform.
Though it can seem odd that Google is betting on such a yet-non-standard format of interface namely SFP+, we can see that it is the format of optics that Cisco has chosen for its Nexus 7000 platform new generation high density 32x 10G card here - which would tend to mean that this type of 10G very short reach interface is not bound to disappear that soon.
I must admit that I wasn't able to collect as much info as I would have hoped on these switche's architecture: no white papers, no presos, well... pretty much nothing.
If someone from Google comes up to me with some additional info on those, that he's allowed to share, I'd actually love that.
Still, while searching the web for extra info, I came accross this: Arastra, a newly born 10G ethernet switch vendor. Those kits are very dense, implement 10G Eth SFP+ (48x 10GBASE-CR, 1U Rackspace) and seem to match Google's requirements. On top of that, I remember having read somewhere that Arastra's CEO, Andy Bechtolsheim former Sun Executive, one of the initial Google investors, is closely tied to Eric Schmidt - again, this is only hypothetical, but there might be some hidden link between Google's Secret switching appliance and Arastra's Kits. I'll post more as soon as I know more, in the meantime, if you have any info I don't, do not hesitate to share :)
Some interesting links when it comes to Ethernet: