However there is nothing indicating that Valve is using LLDB because they want to offer it as a proprietary friendly debugger or that a GPL licenced debugger would cause any problems for developers, that's just deanjo trying to make it into a licence issue.
The actual Valve developer in question has already stated why they opted for LLDB over GDB, which was that they though it would be easier to build a GUI around it, which in turn ties in to what I speculated about earlier, that they (Valve) want to make it as easy as possible for what is mostly Windows based developers to get up and running while developing for Linux, and a debugger which offers a similar GUI and functions very much the same way that they are used to will obviously make it easier.
Now Michael in his eagerness to attack FSF/GNU based software tries to equal this with GDB being 'crap', which again just betrays his agenda. GDB needs to function as a system level debugger, and since neither Linux or *nix for that matter comes with a built in gui, GDB needs to fully function in text mode and this is it's default mode. It's equivalent in Windows, WinDbg does have a simple gui (given that Windows has a core gui) but it was a crude gui back when I was developing on Windows.
There are of course numerous front-ends aswell as IDE's which integrate GDB (including Visual Studio through plugin), so it's not as if you are stuck with pure text mode unless you are developing at low system level. In short, GDB is not 'crap' by any stretch of the imagination, it's also not the 'Visual Studio Debugger' because it has requirements far beyond that, like being able to run and be fully functional in a very sparse environment where you simply can't expect any sort of graphical interface.
Meanwhile the game developers who are used to Visual Studio Debugger will want Visual Studio Debugger, they would likely be just as annoyed with Windows GDB equivalent WinDbg, and this is what Michael tries to turn into 'GDB is crap', again all part of his pro Clang/LLVM, anti-FSF agenda.
So Valve will offer game developers a GUI based debugger which more resembles that what current game developers are used to (again most likely Visual Studio Debugger), and perhaps even an entire IDE in the future, but there's no indication that they will do this in a proprietary manner, and I for one would be very disappointed if they did as I don't see any reason for it.