and it appeares that Google really pays more attention to hyphens than to underscores Here is GoogleGuy's post at WMW:
Yah, I'd stick to hyphens, periods, or commas. Most people seem to prefer hyphens. If you use an underscore '_' character, then Google will combine the two words on either side into one word. So bla.com/kw1_kw2.html wouldn't show up by itself for kw1 or kw2. You'd have to search for kw1_kw2 as a query term to bring up that page. The characters you can use in domain names are pretty restricted: a-z, 0-9, and the '-' character. For subdomains and url paths (stuff after the slash), you've got a lot more flexibility, but I'd recommend keeping it pretty simple. That makes it easier for search engines and users to understand.
There's actually a proposal so that you can encode all sorts of characters in a domain (e.g. CJK--Chinese/Japanese/Korean) but that's a little outside the scope of your question, and I'm not as familiar with the encoding. My rule of thumb is to keep it simple where you can.
Although Y! and Google Directoies using underscores... kinda strange. But in the serps it's pretty obvious - hyphens floating on the top.
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