What's the argument that Fitzgerald's appointment was unconstitutional?

Factual Questions
1

I’ve seen news articles that one of Libby’s arguments is that the appointment of Fitzgerald was unconsitutional, but they never expand on why. Any Dopers able to fill us in on the details?

2

http://www.nytimes.com/2006/02/24/politics/24libby.html?ex=1182052800&en=66270d1c1382c03a&ei=5070

3

Here is an HTML-ized version of the pdf of the brief: http://64.233.167.104/search?q=cache:nBWpG38UKTAJ:justoneminute.typepad.com/main/files/Libby_070608_Amicus_Bork.pdf+Libby_070608_Amicus_Bork.pdf&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a

And here is the pdf: http://justoneminute.typepad.com/main/files/Libby_070608_Amicus_Bork.pdf