Hey Alex
I tried my iPhone 3GS and it worked. Button appeared normal, login process works.
Tried my iPad 2 on my site (I emptied the site cache and restarted Safari on my iPad), and this is what I see:
The text is static and cannot be selected.
SourceCoast login responds the same on my iPad:
If you look carefully, the login text is present, in black. I expanded the page and tried to select the FB login, I could do the LinkedIn and the forgot your password, but not FB. My iOS on both devices is 5.0.1. Your link above is to this conversation.
But I did find that one.