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I think the way leading zero padding is handled for int and float by
format's 'n' code is a bug.
>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'en_US.UTF8')
'en_US.UTF8'
>>> format(12345, '010n')
'000012,345'
>>> format(12345, '09n')
'00012,345'
>>> format(12345, '08n')
'0012,345'
>>> format(12345, '07n')
'012,345'
>>> format(12345, '06n')
'12,345'
When 'n' support was added to Decimal, leading zeros had commas in them:
>>> from decimal import Decimal
>>> format(Decimal(12345), '010n')
'00,012,345'
>>> format(Decimal(12345), '09n')
'0,012,345'
>>> format(Decimal(12345), '08n')
'0,012,345'
>>> format(Decimal(12345), '07n')
'012,345'
>>> format(Decimal(12345), '06n')
'12,345'
Decimal also has the same support for PEP 378's ',' modifier:
>>> format(Decimal(12345), '010,')
'00,012,345'
>>> format(Decimal(12345), '09,')
'0,012,345'
>>> format(Decimal(12345), '08,')
'0,012,345'
>>> format(Decimal(12345), '07,')
'012,345'
>>> format(Decimal(12345), '06,')
'12,345'
>>> 
As I'm implementing PEP 378 for int and float, I'm going to make it work
the same way that Decimal works. For consistency, and because I think
the current behavior is not useful, I'd like to change float and int
formatting with 'n' to match Decimal and PEP 378 for the ',' modifier.
Since I consider this a bug, I'd like to consider backporting it to 2.6
and 3.0, if the changes aren't too intrusive.

I concur with your plan.
BTW, have you checked to see what Java and C# do?

C# doesn't seem to have the issue because they don't allow any modifies
when specifying locale-aware formatting. Specifying a picture seems to
be the only way to get leading zeros added.
Similarly, Java looks to be picture-based.

I agree that Decimal should be consistent with int and float.
I'm not sure I'd classify the current behaviour as a bug, though:
presumably this behaviour was intentional at the time it was
originally coded? I guess backporting *probably* won't do any
harm, since it seems likely that format(x, 'n') hasn't gained
wide adoption yet.
+1 for changing this in 2.7 and 3.1; -0 for backporting the change.
Eric, did you mean to assign this to me? I'm not sure I have
time for this at the moment, though I'd be happy to review
and test when the time comes.

Oops, assigning it to you was an error. I was just trying to figure out
what your userid is so I could add you to Nosy, and I was using
"Assigned To" to find it. I've fixed that.
The current behavior is an accident of the implementation. The
implementation isn't based on anything else, and there was no
requirement to have the output that it does. And as far as I know, there
are no tests that test for the current behavior.
Right now I'm +0 on backporting. What I'll do is fix it for 2.7/3.1 and
see how big the patch is. I suspect it will be a pretty big, invasive
patch. If so, I'll change my backport vote to -1.

This won't get fixed in 3.0 or 2.6. Still not sure about 2.7, but I'm
considering how to fix it there.

Fixed in py3k (will be 3.1) in r71665.

Fixed in trunk as part of r71796. Closing the issue.