[EDIT - Some further investigation...]
OK... OK... So I'll be the first to admit I have been terribly and horribly wrong. And I'll tell you all why....
You shouldn't use display: none for the span - .logo span {}!!! It's not necessarily the wrong way, but its not the best way.
Like I said, I don't know it all... But I do do homework (Yes, I said "doo-doo"). And here's the homework:
So, what to do...
I've check on some reputable sites, and most use "text-indent: -9999px;"
I check out some of their sites in the simulator, and well, yes... ...The web-crawlers pick up on it. [Still Relevant - kinda]
Let's go back to the law, shall we...
The W3C specification states, "lease note that a display of ‘none’ does not create an invisible box; it creates no box at all..." - Which is subsequently not read as by searchbots as there is no box... [EDIT - on screen readers] Unlucky.
From what I see, the text-indent technique is now the staple, stable, cleanest, bestest way of doing what I proposed.
My apologies for leading anyone astray. I am going to correct my mistakes tomorrow.