I just wanted to give a heads up on two nice applications/services running on the iPhone (BiteSMS) or the Android Plattform (ChompSMS)
What they'll basically allow you to do is sending SMS messages over your mobile phone's TCP/IP connection.
The SMS will have your phones own number as the "sender"; address, so people will generally not see any difference in weather or not you had sent it directly via your carrier or using bitesms/chompsms.
The rates are pretty decent:
3 Euros for 50 SMS credits --> 6 cent/SMS
6 Euros for 105 SMS credits (5 "free" credits)
12 Euros for 210 SMS credits (10 "free" credits)
30 Euros for 550 SMS credits (50 "free" credits)
The Software you install usually has the feature of replacing the default SMS application. On Android, this works pretty fine out of the box using the android market. For the iPhone you’ll have to do a jailbreak and install it using Cydia.
The apps allow you to send SMS using your current carrier and just use the extra features the apps will provide
ChompSMS and BiteSMS are developed by the same company and the support was one of the best ones I’ve seen so far. They helped me transfer my credits from BiteSMS to ChompSMS within a few minutes. They also seem to answer email requests almost instantly which makes them a bliss to deal with.
You might want to check the list of supported counties (hint: nearly every country as far as I can see).
I personally have a “flat rate”; data plan on my mobile carrier, but even with the expensive prepaid data plans here in germany, you might end up cheaper than the regular SMS fee using Chomp/Bite-SMS
The iPhone app even features nufty things like this:
ChompSMS looks pretty similar and also features an onscreen keyboard! (which isn’t part of the current Android stable build)