My sister is visiting from Italy. She was trying to send an SMS back home, but apparently you have to know the SIM card number of the phone you are trying to reach, as well as the phone number -- this is on the Vodafone network.
So, I did some quick searching to see if there were some web-based services that allowed this. The two I found that (sort of) worked were Send SMS Now and ipipi. Send SMS now doesn't require an account, but it also doesn't seem to work -- we tested by trying to send to my phone here, and nothing ever arrived. Send SMS Now has an account-based system which requires an email address and sign-up. It also provides free email services and in general works quite while. However, the "free" part is exaggerated: we sent (and received) an SMS message to my phone, but you only get 2 credits for free. You then need to pre-pay your account to get more credits.
With all these difficulties and walled garden type approaches, I'm still convinced that pure IM/email will actually leapfrog SMS here in North America.
By the way, this will be my last post for a while: I'm off getting married and won't be back in action until August 9th.