I've seen the bold too and perhaps you may be right, but everything I've heard and read from those who know more than I do indicated they don't see the separate words in the domain when ranking the sites. Perhaps that knowledge is out of date though.
Either way your domain doesn't play a big part in where you rank. It possibly is a ranking factor, but any benefit is minor. There are better reasons to choose a domain and you can always add keywords in the url, by naming files and folders with them.
The idea of whether Google is going to see wildlifeart and wildlife-art as the same is interesting though. I've seen it debated a lot and always came away from the debates thinking they don't see them the same way. It seems silly though since they do use stemming and there's no reason why they shouldn't be able to.
Of course there's no reason why they can't figure out that domain.com and
www.domain.com are the same domain, but Google sees them as two different domains.
Here's a post from Matt Cutts discussing
underscores and dashes. The post is almost a year old so things may have changed, but as far as I know they are still the same now.
The rationale behind not using the underscore is because it's seen as one word instead of two. I realize we're talking about something a little different, but if you wouldn't want word1_word2 because it won't be seen as word1 word2, I would think the same would apply to word1word2.
I think the technology behind the syntax highlight that bolds words in the results is different from the technology used in choosing where to rank a page based on the url.
I don't know why it should be, but that's been my interpretation of things for awhile.