Cell Phone Encryption?

Black Diamond asks: "I know I'm not up to speed on cell phone encryption, but I was wondering, are there any cell phones that let you handle the encryption from your end of things? Something along the lines of a phone you hook up to your computer to input specific encryption keys for specific contacts, as well as a private key for yourself. Is such a thing plausible, or should you trust the standard encryption that comes on some cell phones nowadays?"