I agree the printf style format strings are not nearly as pretty for dates as SimpleDateFormat. I would like to see a general picture style formatter too, but Java doesn't have one (at least not part of standard Java).
I noticed after I posted that 1.5.7 was indeed recently released. That is unfortunate and I wish that such an important method name as 'format' had received more review and this conflict with String.format had been caught.
And while it would be legitimate to have a breaking change on this in 1.6, the good news is it occurred to me that we can have both.
Naturally I had originally thought that the format string would simply be a format specifier for a single argument (self), but we could instead look for '%' occurring in the format string.
For Date, if it is not present we can use SimpleDateFormat.
For the other types we'll prefix the format string with a "% if one is not present.
And as you say implementing Object.format is a new issue, but I started here because of the conflict.