Custom Post Names Can not include capital letters

[rfrankel]

After upgrading to WP3.1 I noticed none of my custom taxonomies worked with my custom post type. I have been able to figure out what is going on an it has to do with a custom post type being registered with a capital letter in its name. I.e. (set up code removed for clarity):

Will WORK:
register_post_type('posttypename', $args);
register_taxonomy( 'milestone', 'posttypename', $args );

Will Not Work:
register_post_type('postTypeName', $args);
register_taxonomy( 'milestone', 'postTypeName', $args );

I am not sure if this is by design or a bug but it has worked fine up until 3.1.

This is my first bug report so hopefully I haven't made an a of myself.

[greuben]

Dupe of #16600