I think that Privatize needs to go into a category of 'answering a problem that doesn't exist'. The idea is that you log into your Twitter account, write the message that you only want to share with a few people, and post it. Those who are included on the list of VIPs can then log into Privatize to see the message, while everyone else gets told 'sorry, this isn't for you'. Irritating or what?
My first problem with this service is that I have a feeling that all of the people who follow me, being intelligent sensible people are going to think 'this looks like a possible hacked account, no way will I click on that link, I'll check with Phil first.' This then adds to my workload, it doesn't diminish it. It's also going to annoy the people that the message is not for because I'll have to say 'No, it's fine, but it's not for you'. Way to make friends and influence people or what!?
My second problem is that if I know these people then I can send them a dm. Or an email, or contact them on Flickr, or Facebook or LinkedIn and so on. These social networks exist for a reason, so why not use them correctly? Twitter isn't about private messages to selected groups of people - I really don't see the point.
However, if you're interested in trying it out, give it a shot and tell me what you think!