When the format string passed to Printf and friends specifies a width for a value (e.g.
"%10v"), the width is ignored if the value is nil. The algorithm should
probably be changed to convert nil to the string "<nil>", and then apply
width and other specifications just like for any other string value.

What steps will reproduce the problem?
http://play.golang.org/p/tulmfQ5QYE

What is the expected output?
|         x|
|         1|
|      true|
|     <nil>|
|         y|
|     <nil>|

What do you see instead?
|         x|
|         1|
|      true|
|<nil>|
|         y|
|<nil>|

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
Windows 7

Which version are you using?  (run 'go version')
go version devel +aba17f1b93db Thu Feb 07 18:54:21 2013 +0800 windows/amd64