You should read Alistair Reynolds then - it's probably the best (and sadly, probably nearly the only) new hard science fiction there. It's really very good.

If you're not sure, try reading Galactic North - it's a collection of short stories, most of which are set in the Revelation Space 'universe'. It's interesting in that there is no travel faster than c, and people are the usual - grubby and self-serving - no Captain Picards.

Easy solution to stop others reverse engineering the client and writing an open-source one or vice-versa: use asymmetric crypto. It's not really nice, but perfectly technically possible. Particularly, if you're selling one part then you can use a group-based approach, so if somebody reverse engineers one, then you know who did it.

This is a classic example of why the GPL is a bad idea - it's incredibly vague. Besides, if you actually cared about freedom you'd use a BSD or similar license. I only use the GPL if I want to dramatically restrict others' rights, not grant them.

I think that typically new, fancy gases are used. I've seen FM200 (http://www2.dupont.com/FE/en_US/products/FM200.html) used. For example, Internode apparently uses both FM200 (triggered by smoke) and water (triggered by heat). There's a slideshow with some info buried somewhere inside about it here: http://www.computerworld.com.au/article/306254/inside_internode_data_centre

I believe that it's pretty expensive though, and that Internode facility is a very small DC compared to some of the ones discussed in the article above, so I'm not sure what they'd use.