Instead of imitating the look of USD notes and putting pictures of random scientists and silly slogans of the 'in ${whatever} we trust' variety, I would really like to see a design that reflects some of Bitcoin's fundamental building blocks.

For example:

• shapes derived from elliptical curves that the ECDSA algorithm is based on
• watermark based on the bit patterns of the individual iterations of the SHA1 function (160 bits go through 80 steps which could be visualized as 160x80 bitmap)
• prime numbers!
• some reference to Bitcoin script - it's kind of key to the system yet many people are not even aware it exists
• express the peer to peer nature of the network
• symbolic representation of the block chain

I'm not knocking the existing designs but I do feel we could go for a little bit more depth and really revel in the mathematical roots of Bitcoin .