Rumors abound regarding some other manufacturers providing their own proprietary versions of EX processing in their processors and receivers, but I have not been able to substantiate any of this right now. I would note however, that technologically this shouldn't represent much of a challenge to manufacturers as the extra matrix decoding could be applied like any other DSP during post processing of 5.1 channel soundtracks. Both obviously share the same goal, but their approaches differ significantly. ADA has perhaps the most comprehensive solution in that their two processors, in conjunction with one another if necessary or desired, will turn any existing setup into one that will decode DD-EX soundtracks. You would simply connect these line level outputs to the designated inputs on the ADA 6.1, and from there to the power amplifiers for the surround left and right and back surround speakers, or back surround speaker if you choose to use only one speaker for the back surround channel.
Receivers are another story, and here's where the options add up.
The first possibility is that you use a 5.1 receiver that has preamp outputs for the left and right surround channel. In this setup the Ampx4 powers all of your surround speakers, left and right surround plus back surround speakers. This means if all you have coming out of your 5.1 receiver is speaker cables, instead of connecting those speaker cables to your left and right surround speakers you would connect them to the Ampx4. But consider that official THX-EX is initially only going to be available in THX Ultra certified gear, which is expensive. THX-EX also has yet to be announced for any receiver at all.