The PDair aluminum case is very strange. Just wonder if they have actually tested using it.
I don't have a good impression of PDAir's products in general. I once bought one book-style side open leather case and one stylus for my old devices. The layers on the leather top cover of the case began to split along the edge after just 6 months of use! The stylus was worthless as the tip broke off when it dropped from my desk, which has never happened to all other stylii I've had. I couldn't repair them even with those strongest instant glue as they fell apart easily.
Speaking of case, I just wish if ExtremeLimit had made one for this phone. They made very robust, well-designed and stylish cases in the past. They are a bit expensive but are definitely worth the money. But the company seems to have been out of business for long.
http://www.extreme-limit.co.jp/english/index.html
[Correction: just found they have a new japanese site. The link above seems obsolete.
http://www.extremelimit.jp/ ]

Pielfarma is also an excellent choice, but they only have cases for N8 and X6 and not N900.
http://www.pielframa.com/

Another question about the skins. Is it a concern of overheating when the phone is OCed?