Last modified: 2009-02-12 19:45:46 UTC
There's no way to know if the database is locked when registering an account until the form is submitted. The form locked message should be displayed on that page, just like it is while editing a page if the database is locked.
(This is a little annoying when registering accounts on every Wikimedia wiki.)
Alternatively, the "create an account" page should be outright impossible to reach.
Created attachment 4703 [details]
Made an attachement, shows readOnlyPage() on Special:Userlogin, it could probably be made to show it only when your creating an account.
*** Bug 13774 has been marked as a duplicate of this bug. ***
Fixed in r35885 with a change to only show it on signup, not on login/logout.