Another suggestion: I realize you want to keep it pure, but how about adding an extra character as a checksum? It's very easy for the payer to mess the address slightly by writing it down wrong or hearing something wrong. A one character checksum would drastically reduce the chances of sending money to the wrong account.
Adding an additional character of your firstbits address will accomplish the same thing (miniscule chance of a typo), but without ruining the purity.
Example:
One of my firstbits address: 18tkn
The same address, but more protected against typos: 18tknb
The same address, but VERY protected against typos: 18tknbs
Basically, once you own a firstbits address, you also own all of the firstbits addresses beyond the minimum chars necessary to uniquely define the address. And each added character would add in typo protection just like a checksum would (it will probably be a long, long time before any address is created that is only one character off of 18tknbs). It is then just up to the individual handing out the firstbits address how much "typo-resistance" they want to give their address, versus having it shorter and easier to remember.
(besides parsing HTML—ouch).
