Hmm,
XML files don't seem to show-up too nice in the comments...

Another try:
<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
<name>YOUR-PROJECT-NAME</name>
<comment></comment>
<projects>
</projects>
<buildSpec>
<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>
</buildSpec>
<natures>
<nature>org.eclipse.jdt.core.javanature</nature>
</natures>
</projectDescription>

Erwin

Depending on how brave you are, you can just add the Java Nature to the project properties file.

If you open the .project file, and have a look at the XML (make sure Eclipse is closed at this point).

There's a section called <natures>. Just add a line like this;
<nature>org.eclipse.jdt.core.javanature</nature>

And that will magically turn it into a Java project.

To make it compile, you need to add in the following:

<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>

Into the <buildSpec> element.

I just tested it, and you don't even have to shut down Eclipse, if you edit the .project file, using the Navigator view it will magically turn from a simple project into a Java project complete with Build Paths etc.

There you go.

Thanks a lot guys.
I just wonder why eclipse is not supporting this. I could have needed this already a couple of times.
So thanks again for the hints. Will give this a try the next time I am to stupid to do it right in the first place.