Same for Nigerians seeking to transfer millions of dollars to me (if I give them my bank account number).
I haven’t gotten one of these in a year, after often getting several a day. I assume that the spammers realized that the return per period of time — the price of the activity — was less than its marginal cost: the opportunity cost of their time. They have shut down the business and moved to other activities that might yield higher returns.
Economists proposed taxes on mass emailing, but the market seems to have solved the problem. Spam still exists, though; the latest is the email saying I’m the nearest surviving relative of XX [pick some first name] Hamermesh, and if I would just send my bank details to a particular large South African bank, they will wire my inheritance.

A quick review of my Gmail automated spam folder reveals that there are still plenty of people who want to sell me Viagra (no less than eight of the first 20 messages). Perhaps the spammers are still producing the e-mails, but our filters are getting better at screening them?
maybe your spam filters are just getting better.
Daniel, you really should give credit where credit is due and thank your mail admins and others who are fighting to keep the spam out of your inbox.
All that spam, especially the spam for little blue pills, is all still, out there. This screenshot shows the two I got in under an hour earlier today.
http://www.twitpic.com/uiq0v/full
If possible, ask your mail admin if you can get a list of Subjects for mail that has been filters. You may be surprised. Or just ask your readers to peruse the spam folder in any active Google Gmail account.
Filtering, over the last year, has finally made it possible for people like you to feel the problem has disappeared and that is a great thing; however, we mail administrators wouldn’t mind having out role in that acknowledged.
The more likely solution is that your e-mail provider has more effective filters. There is still plenty of Viagra and Nigerian 411 spam.
Ouch, is this how research is done these days?
As all the other comments have mentioned, the spam is still flowing (at the same rate if not more) only it’s your filters that are better.
Your hypothesis is simply incorrect.
I haven’t seen much Viagra stuff but I get a scam or two a day in my spam filters on gmail.
ps: the scam you’re getting (last surviving daughter of…) – that is the same Nigerian scam, just a new story.
I’m still getting the odd Nigerian 419 spam, only not from Nigerians any more. The latest one I got claimed to be from somebody in the US Army who found a large amount of cash in a garbage can when they were rousting out Saddam Hussein and now he wants me to help him deal with it.