Literally, ooh, dozens, perhaps handfuls, of people may have gone into shock at the revelation that 50 Cent isn't keeping it real and has a web person writing his Twitter updates for him. It does raise an interesting issue though. Many celebs and others - brands, CEOs, etc - are rushing to get onto Twitter (largely, I suspect, because it's the flavour of the month). It's a fair bet that more than a few aren't actually Tweeting themselves but employ somebody to do it.
Does this actually matter? On one hand you could argue that as long as the message is getting out then, then possibly not. It also makes it easier, from a PR point of view, to control said message.
However, I'm not a fan of ghosted Twitter accounts. With brands, it's a bit easier as you're speaking as said brand, but it gets trickier when you get into the realms of real people.
If you're replying as, say, 50 Cent then you're opening yourself up to having your celebrity misquoted - or rather having words put into their mouth. It also does a disservice to those who follow the official account only to find out it isn't their hero.
Stephen Fry, Jonathan Ross, Phillip Schofield and many many other celebrities seem to have no problem being themselves on Twitter. If anything, the way they use the service has enhanced their personal brand.
Ultimately, if a celebrity isn't that keen on the idea of Twitter or doesn't want to Tweet, it'll probably do more harm than good if they just pass it over to somebody else. I'd certainly feel uncomfortable in working on a ghosted Twitter account.
As for corporate 'important people' Twitter accounts (eg council bosses, company CEOs) ghosted accounts, I'd say, are even more of a no-no. It implies your brand or head honcho or whoever is on Twitter for the sake of being on there and can't really be bothered.
It's much like those corporate blogs where the head of the organisation or somebody else in a position of importance either decides they want to blog or get told they should blog. What follows is usually about two posts before they lose interest and make no attempt at engagement. It's easy to see that being translated onto Twitter.
The only way that this may be acceptable is if the account was set up in the person's name but it was made clear from the beginning that this was an account for the person's 'brand' rather than the individual themselves.
Transparency, as ever, is the key aspect when you're dealing with social media.
Ultimately, if whoever it is isn't into Twittering or blogging or whatever you can't make them, and it may do more harm to the individual's brand in the long term that the brief 30 seconds of kudos they'll get from the online community.