It's good to see Tim blogging again. Hopefully he'll keep it up this time (nudge, nudge, Tim). At any rate, I'll blame his lack of practice for his post "Craig urges overkill, XmlSerializer sky not falling". Either that, or he had a high temperature/blood alcohol level. :)
Craig got caught in a very particular set of circumstances. First, he started with WSDL. Then he hand-wrote his serializable types. Then he followed his preferred set of rules for ordering members of those types alphabetically.
The implication here is that unless you're doing all those things, you won't have the issues I describe in my post. This is simply not true. The problem I describe is an issue for anyone who does not explicitly control the order of serialization of their web service-visible types. Period. While I wouldn't exactly say the sky is falling, this is definitely a Big Deal.
I think the reason you don't see this occurring as a problem more often in the wild is that people tend to write .NET clients for their .NET web services, and XmlSerializer doesn't care about order. Or, more generally, schema validity. But if reach is important to your web service, you should.
I actually talked with Tim about this on the phone, and it came out during the conversation that the problem is even worse than I first thought. I had detected the issue with return types, but the fact of the matter is that if you reorder your type members for either input or output parameters, you change the generated schema in the WSDL. And that's a breaking change (from a schema standpoint).
Which really was my whole point: you need to be EXCEEDINGLY careful with your types unless you implement IXmlSerializable or use XmlElementAttribute.Order. Reordering type members is just too easy for a developer to do without thinking about it, and isn't the sort of thing that's easy to catch as critical even if your team reviews all source changes.
As Tim points out, implementing the read side of IXmlSerializable is a royal pain most of the time (you'll note I only said "consider" using it), but XmlElementAttribute.Order is pretty easy. Of course, it's only available in .NET 2.0. :p