What percentage of network traffic comes from botnets?

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I was recently asked what percentage of our network traffic can be attributed to botnets and if 60% was a reasonable figure.  This is not something that I have personally investigated (ie, I don't have the numbers to support my theories).

According to a lot of articles that I have read (and knowing from experience), spam has undergone a transformation in the past year.  Professional spammers with netblocks used to account for a big chunk of spam, that has now shifted into networks of zombies.  The question is how much has that shifted?

I'd say at least 50% of network traffic is mail coming from botnets.  I think a case could be made that it is as high as 60%.  What do the rest of you think?

Update: This was in one of the links in Enemieslist's Links Roundup on Jan 23:

Sophos noted that up to 90 percent of all spam is now relayed from zombie computers hijacked by Trojan horses, worms and viruses and under the control of hackers.

So there you go, that's one source.