As a domainer AND affiliate marketer, I think keyword domains are extremely valuable. There are, however, several things to consider.
I am not a fan of hyphenated names. They may have their advantages for SEO
and be available for registration fee (there is a reason for that), but tend to lose returning or word-of-mouth traffic to the non hyphenated version (assuming one exists). Often they also tend to look like this-name-cost-eight-bucks.com, which makes you wonder how well they will actually convert. Image is important. Though I will admit I may be biased as a domainer/marketer first & consumer second.
G has been able to parse keywords in non-hyphenated names for some time now. I would try to get a non-hypenated keyword name if possible.
Some non-hypenated .coms get enough direct navigation traffic to make a grown man cry. This is reflected in their trading prices, though. Hand registering a "traffic" name is not a likely option as this point.
Using the cell phone example, cellphones.com would be the first choice. Alas it is taken & the seller wants 2 million for the name or just isn't selling (I wouldn't). I would then check overture for keyword phrases that people are using when searching for cell phones so that I might get some type-in traffic in addition to any organic or paid traffic.
You will probably find all of the keywordcellphone.com's are taken as well. D@mn domainers thought of that already too! From there the choices are to:
1) buy a keyword name from whomever saw a market & registered it before you did
2) register a short "add on name" i.e. uscellphones.com or cellphonesplus.com
3) buy the keyword name in an alternate extension like .net, .info, or .us. This will probably still cost some money, but only a small fraction of the .com version.
At this point, I would rather spend a few hundred to a few thousand on a good, authoritative, name in .com, .net, .info, or .us than invest any time in a hypenated name & then wonder if the conversion results could have been better.
IMO, a good name can earn more for you in increased conversions & traffic than it cost to acquire... a sound investment if you hope to grow. Who would you buy from... this-name-only-cost-eight-bucks.com or keyword.us ?
It should be noted that non .com names will not get much type in traffic, but neither will hyphenated names or keyword names with an extra word tacked on i.e. cellphonesplus.com
In regard to subdomains (keyword.domain.com vs domain.com/keyword), I would choose the domain.com/keyword structure every time. It is just much easier to administer particularly for database driven sites. Some CMS's are able to run multiple subdomains off of the same database - but I can't see it as being worth the effort in most cases.