Blimey - that's a good 'un! I've seen several cracked rims and cracks around spoke nipples but I've never seen that before.
I was out on on one of my forum rides and heard a slightly thudding in time with each revolution of my back wheel. I got off and took a look and discovered a small bulge in the tyre which was catching on my mudguard. When I got home, I took the tyre off to investigate and discovered that it was delaminating from the inside, a bit like that rim cracking from the inside. At some point the tyre would have failed and the tube would probably have exploded.
I recommend that all cyclists should carry out regular inspections of their bikes. Don't put off fixing any problems that you find - I've had all sorts of problems when I did that, ranging from a failed handlebar stem, to a destroyed rim and an exploding tyre!