Type 'g' formatting for Decimal instances doesn't behave in the same way 
as for floats when an explicit precision is given.  It should strip all 
trailing zeros from the result:

Python 2.7a0 (trunk:75309, Oct 10 2009, 13:44:18) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import Decimal
>>> format(Decimal('123.00'), '.6g')  # expect '123'
'123.00'

(When no explicit precision is given, Decimal formatting with type 'g' 
tries to preserve the information about the exponent of the Decimal 
instance whenever possible;  this also differs from float formatting, but 
it's intentional.  This should probably be documented.)

Here's a patch for trunk.

> It should strip all trailing zeros from the result:

Hmm.  Thinking about this some more, I don't think this is true:  format() shouldn't be throwing away significant information (in Decimal 
the number of trailing zeros *is* significant information) unless that's 
necessary to fit the result into the given precision.

So I think the current code is correct.

However, we could do with documenting the precise rules being used for 
Decimal formatting somewhere.

I would like to think about this one for a bit before it moves forward.

[Mark]
> So I think the current code is correct.

I agree with this. Currently the 'g' format is like to_sci_string()
with the added possibility of adjusting the number of significant
digits. It's probably hard to come up with a better way to handle
this for Decimal.