Thanks Adam for the information about Immix. They do seem to have plenty of roaming agreements, but it's not clear if anything other than T-Mobile and AT&T work for thie prepaid:
GSM Coverage Maps | United States | Keystone Wireless LLC (Immix Wireless - Pottsville, PA) | Network Information
The real Immix postpaid even roams on Movistar Mexico (amongst many other countries) - if they added that as well to the Simple Calling card they would have a real winner.
As I mentioned above the eKit ad on ebay did list a 50c connection charge in the examples in the footnote to the rate table, but it's certainly misleading and easy to miss. Hope they haven't forgot to mention anything else (like monthly 911 charges... ).
Having said that, I have ordered a couple of these as I may be in Canada later in the year plus it gives me extra coverage in the US. Got a couple of nice consecutive Hollywood numbers (one of them ends in two sets of double digits) according to the email - haven't seen the SIMs yet as I'm in Mexico right now.
How did the activation process go? Any catches there? Any warnings about when you must reactivate it again to keep it active? Any online call records or account balance?
BTW, a sort of workaround for receiving international SMS might be Google Voice - that is supposed to be able to forward SMS to US numbers, and the Simple Calling is supposed to receive from US. So you could give the GV number to a non-US contact. (but, when you reply to their SMS it will come from the SC phone number, unless you go online to GV to send it for free). Also, GV are not giving out any new accounts right now (I have one).
I noticed 0044 (UK) are also selling these (at higher prices). In their notes, they say the problem with incoming SMS is only from South Africa. So sounds like it may be variable depending on the country, worth experimenting with.