It did ask for robots.txt (gasp) and has 3 IPs with rDNS best I can tell.
Twitterbot/1.0
199.59.149.164 -> spiderduck01.dmz1.twitter.com
199.59.149.165 -> spiderduck02.dmz1.twitter.com
199.59.149.166 -> spiderduck03.dmz1.twitter.com
But I've seen it crawl from all these too:
199.59.149.1 -> r-199-59-149-1.twttr.com
199.59.149.2 -> ditto, etc.
199.59.149.3
199.59.149.4
199.59.149.5
199.59.149.8
199.59.149.9
However, they have another variant that only does HEAD requests and I've seen on the following IPs:
Twitterbot/0.1
199.59.149.3 -> r-199-59-149-3.twttr.com
199.59.149.1 -> r-199-59-149-1.twttr.com
199.59.149.10 -> r-199-59-149-10.twttr.com
