@evildictaitor:It's that way in the documentation. But since:

1) There's only 2 places in the source that uses AppendFormat and they all in the form of FormatString + single parameter
2) After the change the compiler no longer complains it cannot find method matching that signature.

I'd think the only possible reason is that the CF runtime have a problem in matching methods that uses "param" parameters. Param parameter is only available on .NET 3.5 onwards, maybe he's got devices running .NET v3.0 or below.