If we can crack a 1024 bit key today and computing power doubles every year, then we could crack a 2048 bit key in about a thousand years, since the key space is 2^1024 times bigger than for a 1024 bit key. The thing is, our ability to brute force keys appears to be increasing faster than Moore's Law. Assuming a doubling rate of one year, we would only be able to brute force 1 more bit per year, 10 bits per decade. The largest cracked key size is increasing much faster than that. Nightcrawler said that 512 bit keys were cracked in 2002 and we're close to cracking 1024 bit keys now. That's an increase of almost 500 bits per decade, which is why 2048 bit keys won't be safe for more than 20 or 30 years, and that's assuming no disruptive technology comes along.