Thanks for suggesting this idea. One of the reasons that clients currently need to select their hosting plan via the Services > My Services page before the login to cPanel button is available, is because a single client can have multiple hosting accounts with you across different servers. They need to choose which account to login to.
I'd be interested to hear your opinion on how the single sign on link would handle a client with multiple accounts. Also, might it be confusing to clients to have two places to access cPanel?