5.8 petaflops, or 5800 teraflops...
on pretty much all recent archs with vector units, one 32-bit INTOP = 2 single precision FLOP
one bitcoinhash is ~6.35k x86 INTOP
450G hash/s * 6.35k INTOP/hash = 2858T INTOP/s
2858T INTOP/s * 2 FLOP/INTOP = ~6700T FLOP/s
I asked the admin and he responded that the conversion used on the
http://bitcoinwatch.com/ page is from this very thread! The site uses the formula from above (after correction for the typo).
The page simply uses the following assumptions/estimates:
1 INTOP = 2 FLOP
1 hash = 6.35K INTOP
1 hash = 12.7K FLOP
So the hashrate in TeraFLOP/s is simply 12.7 times the hashrate in Gigahashes/s.
For example as I wrote this: 11,558.55 Gigahashs/s * 12.7 TeraFLOP/Gigahash = 146,794 TeraFLOP/s = 146 PetaFLOP/s
So it is true.

I wonder how much electricity it is. Wait a second for the edit.
EDIT: So, assuming everyone is mining with 5970s, the most efficient, it would mean that we got a 2.3 Mhashes/joule -> 13,300,000 Mhashes/x -> 5,782,608 Joules
3.6 mJ = 1 kWh -> ~1.6
kWh?!!!
I most be doing something wrong. D: Ok, it is 1.6 kWh per second. So s*m -> 3600 * 1.6 = 5,782 kWh.

Assuming 15 cents per kWh that is an expensive bill for just one block. D: