Why I think 10G-T will be the dominant form of 10G Ethernet
Today there are basically two options for 10G Ethernet products and interfaces, 10G-T (standard Ethernet ports and relatively normal Ethernet cables) and SFP+ (pluggable modules mostly using fiber). Historically SFP+-based products have been the dominant ones and some places have very large deployments of them while 10G-T seems to only have started becoming readily available recently. Despite this I believe that 10G-T is going to be the winning 10G format. There are two major 10G-T advantages that I think are going to drive this.
The first advantage is that 10G-T ports are simpler, smaller, and cheaper (at least potentially). SFP+ ports intrinsically require additional physical modules with their own circuitry plus a mechanical and electronic assembly to plug them into. This adds cost and it also adds physical space (especially depth) over what an Ethernet RJ45 connector and its circuitry require. In addition 10G-T is pretty much just an RJ45 connector and a chipset, and the hardware world is very good at driving down the price of chipsets over time. SFP+s do not have this simplicity and as such I don't think they can tap quite this price reduction power.
The second advantage is that 10G-T ports are backwards compatible with slower Ethernet while SFP+ ports talk only with other SFP+ ports. The really important aspect of this is that it's safe for manufacturers to replace 1G Ethernet ports with 10G-T Ethernet ports on servers (and on switches, for that matter). You can then buy such a 10G-T equipped server and drop it into your existing 1G infrastructure without any hassle. The same is not true if the manufacturer replaced 1G ports with SFP+ ports; suddenly you would need SFP+ modules (and cables) and a bunch of SFP+ switch ports that you probably don't have right now.
In short going from 1G to 10G-T is no big deal while going from 1G to SFP+ is a big, serious commitment where a bunch of things change.
This matters because server makers and their customers (ie, us) like 'no big deal' shifts but are very reluctant to make big serious commitments. That 10G-T is no big deal means that server makers can shift to offering it and people can shift to buying it. This drives a virtuous circle where more volume drives down the cost of 10G-T chipsets and hardware, which puts them in more places, which drives adoption of 10G-T as more and more equipment is 10G-T capable and so on and so forth. This is exactly the shift that I think will drive 10G-T to dominance.
I don't expect 10G-T to become dominant by replacing existing or future enterprise SFP+ deployments. I expect 10G-T to become dominant by replacing everyone's existing 1G deployments and eventually becoming as common as 1G is today. Enterprises are big, but the real volume is outside of them.
By the way: this is not a theoretical pattern. This is exactly the adoption shift that I got to watch with 1G Ethernet. Servers started shipping with some or all 1G ports instead of 100M ports, this drove demand for 1G switch ports, then switches started getting more and more 1G ports, and eventually we reached the point we're at today where random cheap hardware probably has a 1G port because why not; volume has driven the extra chipset cost to basically nothing.
Update: The reddit discussion of this entry has a bunch of interesting stuff about various aspects of this and 10G Ethernet in general. I found it usefully educational.