I bought and tried ion energy nanoguard chip (http://www.air-boom.com/nanoguard.htm) on the back of n900 cover. I didn't stick it just simply put my n900 on top of it. It did increase signal but not greatly. In my case up to 2 bars. I also compared the signal with or without nanoguard several times. Using it improve the signal slightly and the heat when using 3.5g internet was also reduced as advertised .

However, i decided not to put it on my n900. Why? It's quite ugly on the phone and thick (not like a thin signal booster sticker) and only work best on naked n900 without external casing. I happen to use otterbox casing, so it doesn't go along together well. If i use pouch maybe i will consider using it.