Weighted voting is already implemented on the blockchain, but you can't do it on Steemit.com. I've been looking into voting power lately; follow my blog and you'll see a comprehensive analysis of voting power come out hopefully on Monday.
The short answer: You can't do weighted voting on Steemit.com, but it doesn't matter - if you vote so much that your voting power drops, you end up having the same total influence as if you only voted 25 times per day.