A WebmasterWorld thread has discussion around using commas in the URLs as opposed to hyphens or other characters. For example, this URL would be seroundtable.com/commas,in,urls instead of what it currently is, with hyphens.
The reason why this person wants to use commas is because of SEOs misunderstanding basic concepts because SEO is deeper than what most read.
The issue here is that Google said hyphens are better than underscores in URLs, so some took that to mean underscores are bad. They are not bad. So instead of going the hyphens route or even hyphens route, they want to go to with commas in URLs.
Personally, I would never use commas in the URLs. Why? Well, I am sure they would get indexed fine but they can also be used for encoded URLs. As one member pointed out in the thread, "Thus, only alphanumerics, the special characters "$-_.+!*'(),", and reserved characters used for their reserved purposes may be used unencoded within a URL."
In any event, if you do have these characters in your URLs and they work for you, stick with it. As another member said:
I used commas extensively back in 2004 on a few of my websites. I've had no problems with them whether they are /key1,key2/ or /key1,key2.html
Those pages still rank, but they are now six years old and have backlinks.
If you are looking to find out about SEO-worth of commas, there is none. Hyphens themselves do not give you an 'edge', they just reduce your problems. Commas can produce problems so should be avoided.
But I doubt any SEO would opt for this in 2010.
Forum discussion at WebmasterWorld.