I was under this same mistaken impression (and made a similar comment in https://bitcointalk.org/index.php?topic=102756.msg1128037#msg1128037)  until I took a closer look at the blockchain layout.  The blockchain doesn't use decimals to represent bitcoins, it uses integers representing satoshis.  The bitcoin representation is just something that the client program does to display the value (the client takes the number stored in the blockchain and places a decimal 8 places to the left before displaying it.  This means that we can't just "add more decimals".  We would have to change what the number in the blockchain represents.  There would be a upgrade time period during which some people might still be running software that used the old representation while others use software with the new representation.  This would be a problem (one program sending 100000000 thinking that it is sending 1 BTC, and another program thinking that the amount it is receiving is only .01 BTC), and would require a hard fork in the blockchain.  There are ways to deal with the situation, but it isn't as simple as "just add moar decimals!"