I distinguish this by saying that Social Security and Medicare, as much as we like to be told otherwise, are social programs that are funded by general revenues. Yes, there are FICA taxes that supposedly support these programs, but those funds go directly to the federal treasury and Congress can do what it wishes with the overage, and fund underpayments from general revenues as well. It's a total fiction that SS and Medicare are somehow unrelated to general tax-and-spend powers of the federal government, or that they are independent from the budget. Calling them "off-budget" doesn't make it so.

As for whether the mandate is otherwise constitutional, it isn't. It requires you to pay a third party for services that you might not otherwise want. That's not a tax -- the only money going to the government is a fine if you don't buy the policy, and that wouldn't satisfy your obligation to then buy insurance. It's not regulation of commerce, because failing to buy insurance is no less engaging in commerce than failing to buy the creme brulee last night at dinner. If interested, I'd refer you to a couple posts of mine on this:

http://marquesletters.wordpress.com/2009/09/22/the-health-insurance-mandate-a-policy-in-search-of-a-power/

http://marquesletters.wordpress.com/2009/09/25/the-health-insurance-mandate-is-it-slavery/