Well, Roger is certainly a knowledgeable guy, but I think your question wasn't entirely clear. You didn't ask specifically about SEO and the way you posted your question already contained the answer:
Very often I want the site identity to be a logo, this of
course means I either need to make use of the img tag or background
CSS rule. So far I've always just gone with img because I consider the
logo to be an image in its own right rather than a background styling.
Depending on who and how you ask the question, you will get different answers.
- What is the most semantic way to mark up my logo?
answer: probably an image
- What is the best way to markup my page's main title?
answer: use <h1>.
Want a special font for that h1? use some image replacement technique.
The main question in this thread is, if I'm not mistaken, whether or not using an image replacement technique can have a negative effect on search rankings.
Google is clear about this: do not show visitors something else then the google bot.
Make pages for users, not for search engines. Don't deceive your users or present different content to search engines than you display to users, which is commonly referred to as "cloaking."
So using hidden text full of mortgage related words while showing the visitor something about toys (or visa versa) is wrong. But using an image replacement technique to show a nicer "Myincrediblesite" header/title? Why would that be wrong? The visitor sees exactly the same as the search engine. Google can not and will not penalize that. Might as well forbid CSS altogether and only allow plain text sites to be ranked
There's a lot of talk about this issue on the
google webmaster forums.