Question: I'm searching for something that works like Twitter but for a closed group of users. I also want a SaaS offering, nt some software (e.g. status.net) which we have to install ourselfs.

I'm aware of

* present.ly - but they seem not to support any "payed" accounts (you get what you pay for)
* yammer.com - seem to have questionable business ethics regarding data ownership etc.

I thought there would be hundreds of other similar services but so far i found nothing. Any Suggestions/Experiences?

Answer: 
I've been searching for this for a couple of days, and found status.net the best solution so far. You mentioned it in your question, have you checked their hosted solutions?

E.G. <http://mozilla.status.net/>
Status.net does seem to provide a [hosted service](http://status.net/enterprise-network).