well, not saying that I believe wright's claims (or gavin) but everyone makes mistakes. a little spelling mistake won't be a problem if he gave the community a solid proof.
...though on keyboard the key "i" is quite far from "a".
It is more than a simple spelling mistake. Since he also showed this same mistake in one of the scripts that is visible on his blog post, he could have easily used this "mistake" to trick someone by loading data from another file that the misspelled "signiture" variable was pointing to while the observer assumed that the data was from the variable "signature", which also appears in the same script correctly spelled. They would be two different variable to the computer, but would be something a casual glance through the code to an observer exciting he is meeting the real Satoshi might miss.