The specs for the new retina iMac claim it supports an external display @3840x2160. But: 1. Will it be 60Hz? 2. Only one such display, right? Thanks.

No chance. The I/O is too slow - you'd need a mess of cables. As others pointed out, putting the computer inside the monitor bypasses this and lets the display driver interact directly with the display rather than sending the signal through a port and cable.

Can anyone tell me please will my 2013 Mac Pro handle two such new iMacs just as external displays at 60 Hz? Thanks.

Most likely, your 2013 Mac Pro won't be able to do that because a single cable / single bus DisplayPort v1.2 cannot provide the bandwidth for 5k (DP v1.2 provides about 17.3Gbps which is insufficient).

Unless Apple provide a multi-cable solution for the 2013 Mac Pro when the 5k Thunderbolt display comes out, you won't be able to take advantage of 5k displays on it.

My prediction is that we'll see an updated Mac Pro that also uses the Haswell Xeons (2013 Mac Pro uses last gen Ivy Bridge) and also DP v1.3 (which can support 5k displays).

Anandtech has the following message in their story:

Update: And for anyone wondering whether you can drive the 5K display as an external display using Target Display Mode, Apple has confirme that you cannot. http://www.anandtech.com/show/8623/hands-on-apples-imac-with...

These aren't monitors, they're full computers. You can't even use them as external monitors like the old iMacs could.

Why would you do that?