Sorry if this is a silly and/or stupid question but... Will there ever be, or would it even be possible to have a PHP.NET? Or have I got the wrong end of the stick?
Actually it exists. It's not called PHP.NET, but Phalanger(http://phalanger.codeplex.com). It compiles PHP code into MSIL instructions.
It allows to run PHP application in so called legacy mode (and you can benefit from a faster running time since it's compiled), and if you want to use CLR features that are not present in PHP you can turn on PHP/CLR compiler extensions.
There is also so called Pure mode where PHP langauge is modified to behave more like C# or Visual Basic.
I think combining PHP and .NET is great idea, since you can make the best of both worlds. From PHP it would fast development and deployment process, steep learning curve and there are lots of great PHP open-source applications. On the .NET side, verifiable ISO standartized environment, peformance, managed secure code, advanced development tools,...