Now, possibly this question will be irrelevant in a couple of years' time if we all move to flatrate mobile data (including roaming), but for now, I'm surprised it's an issue that hasn't cropped up more often.
I've just got a press release from LogicaCMG about their web compression ("optimisation") product which is essentially a proxy server which squeezes down web images & generally monkeys around with content to make it render better on a small screen. No, this isn't a particularly new concept, various others have done it in a similar way, or via on-device client software like the Opera Mini client/server approach. Yes, there's also a bunch of question about whether I, the user, am happy (or even know that) my expected pristine Internet experience is being filtered & squished & re-ordered according to what the network thinks I'd prefer, if I can get it a bit quicker. That's a topic for another day (for example, how does it know what screen resolution I can display? does it adjust for what browser I'm using? what happens with Flash and AJAX and all that good stuff?)
But how about the question of what this - and other techniques, think caching or multicast - means for the billable traffic flow. Am I billed for the actual bits that go over the air to my phone? Or the raw pre-optimised content going into the server? What about if some of the content is cached locally at the base station? do I & multiple others pay for that as well? (various people have suggested that this is an option). Or if some stuff gets multicast? Is there any "overhead" that I don't get to use or even see - am I paying for that too? No big deal if it's flatrate of course, but I pain if it's €10 a MB on international 3G. Or if it's a mid-size bundle, how do I know my consumption is being judged accurately?
I'm reminded of my Yahoo mail. I know that if I get a 3MB mail attachment with a 2-line text email, for some reason Yahoo claims that I received 4MB or 4.5MB in total into my inbox. Eh? As I'm not paying for it, I'm not really bothered. But I certainly would if a mobile operator tried to pull the same trick & then charge me by volume.
Now, lots of phones have a data "meter" which shows total traffic in & out - but it doesn't generally split this by port number, so you don't know what's going to a "free" application (from the data traffic perspective) like MMS or even the operator portal site, and what's charged at off-portal rates.
I would have thought a 3rd-party solution for this sort of thing would be highly relevant for applications like push email, especially for businesses whose staff roam a lot. Do you have an audit trail for your roaming traffic?