Unable to rename a file with 'mv' if a file exists with the same spelling, but different capitalisation

Reproduction steps speak louder than words: (this is on a karmic laptop with ext4 as the filesystem)

-(cmsj@kiryo)-(/tmp)- mkdir casefail
-(cmsj@kiryo)-(/tmp)- cd casefail
-(cmsj@kiryo)-(/tmp/casefail)- touch foo{,1}
-(cmsj@kiryo)-(/tmp/casefail)- bzr init
Created a standalone tree (format: pack-0.92)
-(cmsj@kiryo)-(/tmp/casefail)- bzr add
adding foo
adding foo1
-(cmsj@kiryo)-(/tmp/casefail)- bzr commit -m "add files"
[10850] 2009-08-11 16:34:39.284 INFO: Committing to: /tmp/casefail/
Committing to: /tmp/casefail/
[10850] 2009-08-11 16:34:39.285 INFO: added foo
added foo
[10850] 2009-08-11 16:34:39.285 INFO: added foo1
added foo1
[10850] 2009-08-11 16:34:39.343 INFO: Committed revision 1.
Committed revision 1.
-(cmsj@kiryo)-(/tmp/casefail)- bzr st
-(cmsj@kiryo)-(/tmp/casefail)- bzr mv foo1 Foo
bzr: ERROR: Could not move foo1 => foo: foo is already versioned.
-(cmsj@kiryo)-(/tmp/casefail)-

This ought to be valid, and I am able to "bzr add" an unversioned file called Foo.