I've been politely asked to leave 3 different shared hosting companies. Most advertise unlimited disk space and bandwidth, but they never offer unlimited CPU cycles or disk I/O. For the past two years, I've been hosting with Rackspace Cloud (Cloud Sites, Cloud Files, and Cloud Servers).
Cloud Sites is nice because it scales with demand, and there is no upper limit to CPU cycles. The one caveat is that the databases don't scale (basic master/slave setup), so you'll have to go heavy on the caching for DB based sites. This tradeoff is worth it to me because I don't have to worry about server administration or scalability.
http://www.rackspacecloud.com/?id=605