The line is technically only straight on Log plots, its a exponential formula that appears straight when the y-axis is log. I personally don't ignore any relationship that has an R squared of >0.9 . Can it be wrong? Sure. Does the trend show a close relationship of the two variables, you bet.
edit: accidentally wrote "logarithmic", now fixed
Using all data since the inception of MtGox gives and Rsq of 0.879, still very impressing:
https://bitcointalk.org/index.php?topic=470453.msg5739610#msg5739610I like the wallets squared model better. It actually explains something. You do know that an exp(exp(t)) model actually fits better, with higher R^2, right?