Why would you want keystroke playback in Gmail though? I'd rather have "undo send", among other things. (yes it's doable, just add a small delay to the actual delivery -- I usually realize my mistake within a few seconds)
While they are at it, they could do the same with email (delayed send for certain tags) that you could easily cancel the sending. Also, "did you mean to cc everyone on your list, this is frowned upon" type prevention :)
The thing is - decisions at Google are all made in a data-driven fashion. So "this random person I met on the Internet wants us to implement keystroke playback" doesn't carry much weight. "Someone on Hacker News suggested GMail and GDocs implement keystroke playback, and then 900 people upvoted it, and then it got picked up by Reddit and Del.icio.us, and then the NY Times wrote a story about how it's the next big innovation" would probably be pretty convincing, though. So if you want it, convince other people it's a good idea, and convince them to express their support of it. Somebody at Google will see it eventually.