Qt was not available under an OSS license, originally, which is why GNOME was born in the first place.
There are clearly some legal issues that could pop up around Mono if they ever hit the jackpot - just like what happened to Java when Android sales went crazy. However, I doubt that Mono is ever going to be that big, which means it's probably safe.
There are three open-source implementations of .Net I am aware of and Microsoft hasn't moved against any. Contrast that with Sun/Oracle who control the sole complete open-source implementation and have moved to destroy all other relevant ones (Apache Harmony and Dalvik).
Strange as this may sound, Microsoft has been much more open here.
Miguel's reaction to this whole thing strikes me as someone cheering about how great a tree is doing while the forest burns down around them. That tree is pretty awesome, but without the rest of the forest it's going to get mighty lonely.
After 7 years, this technology proves its value, by offering something unique (portability among all desktop *and* smartphone platforms). He has every right to be excited about this - wouldn't you be, were you in his shoes?