OK, this is fantastic idea.
If you use Internet IM a lot, you'll know the benefits of having multiple identities. I also use Yahoo! Mail Plus, which gives me "disposable email addresses" linked to my main account. So I can use the addresses (or in this case phone number) for people I don't really know, or that I think might send me loads of spam. If any of them get over-spammed, I can simply bin that address, safe in the knowledge that my friend can still reach me on my "proper" email address.
That said, this "Tossable Digits" thing looks a bit pricey - $5 for a month, and you get some clunky fixed-line + 5 digit extension number. And it won't cope with SMS by the look of things. By comparison, Yahoo! charges something like $20 a year for their whole "Plus" service, including a whole bunch of features.
This should be offered as an operator service here. I'd gladly pay, say, $2-3 per month for a spare, bin-able mobile number - which should get a huge gross margin as it really only needs a number-translation server somewhere. At the moment, I am very wary of putting my mobile number down on any untrusted mailing list for fear of spam & unwanted calls, but if I had the confidence that I could consign the number to the great spam repository in the sky, I'd use that rather than my fixed-line.
One last point - it's another one in the eye for those Luddites who insist we'll only have one device / number / bill / identity. Not only are you wrong, you're getting more wrong by the day.