@all: This case (except for RonStrilaeff’s reply) has been discussed 4 months ago and the link in Network admin is still broken, leading to “Access Violation” error message. Haven’t you fixed this yet / updated Akismet plugin for such long time?
@ronstrilaeff: No way, their going to do this, because this is bad. Most people uses WordPress Network to manage own network of blogs from one general dashboard (with registration of new sites disabled by default). But think about those, who run sites like WordPress.com, which are open for registration of new sites. Admins of such networks for sure require, that end-point users enter own Akismet API key as they don’t want hits from user sites to enlarge hits counter of their own Akismet account. Especially, if they’re enterprises and had to pay for API key and will have to pay extra in pay-per-call mood, once their network overall hits count passes certain limit (of 100k hits per day, if I’m not mistaken). So for such installations having abillity of entering different API Keys per different sites is surely most welcome.