Just tested with a temporary anonymous only global block (see URL for log entry), trying to create account on Commons and it.wiki:
You do not have permission to create this user account, for the following reason:
Your IP address has been blocked on all wikis.
The block was made by Vituzzu (meta.wikimedia.org). The reason given is test.
Start of block: 18:26, 12 September 2012
Expiry of block: 18:41, 12 September 2012
This is counter-intuitive, we currently have an Ukraine IP range globally blocked because of many spambots and this prevented account creation from wannabe Wiki Loves Monuments participants, with little clue about everyone involved about the reasons.
Bug 15273 suggests a solution but the behaviour might just be changed/fixed.