And from what I gather even their new .NET licensing terms are designed to leave you on the hook.
(1) Microsoft adopts MIT license for .NET, a perfectly standard OSS license. Many people leave it at this, but MS additionally makes a "patent promise".
(2) Blog site reads the patent promise, notes that for most use of the .NET OSS you're covered by the patent promise, but there's apparently one particular case (where you write your own alternative .NET runtime/fx that's incomplete) that doesn't appear to be covered by the patent promise.
(3) Slashdot summary makes the leap to say that MS is "undecided about suing" users of its OSS.
(4) Burz makes the leap to say that this is actually "designed to leave you on the hook".
There are quite a few unjustified leaps in there. Burz, I wonder if you'd say the same about all OSS software that's licensed under MIT or BSD but which lacks a patent promise? Because such software would be in an even weaker state from your perspective than Microsoft's OSS .NET.
(disclaimer: I do work for Microsoft, and I did generate some patents for them, and I'm an engineer not a lawyer).