Straight Talk has launched a $60/month plan includes unlimited domestic voice, messaging and data plus unlimited international calling to most landlines in 32 countries (PDF list of covered countries) and to cell phones in Canada, China, India and Mexico.
Straight Talk provides a web page where you can enter a number and see if it's covered.
Surprisingly, many countries that have large US expatriate communities including Australia, El Salvador and the Philippines are not covered.
Unlike some of its competitors, including Virgin Mobile and Red Pocket, Straight Talk does not support international direct dialing (+1 - country code - city code - phone number). Instead you have to dial an 11 digit access code, wait for a prompt and then enter the number your are calling, which is definitely less convenient.
Straight Talk has never supported international text messaging except with its Android phones that run on the Sprint network. There's no mention of messaging on the International Plan page so I suspect that none is included.