We were first, so they copied us. :-)
Justin's right that this is the risk you run with off-the-shelf software.
Both sites use the same software platform - stackexchange.com However, one of the big differences is that in this site the hosts are well known successful entrepreneurs and they also do take the time to answer questions all the time - Thanks Jason and Dharmesh!
Have to laugh. My company Page Mage is featured today on killerstartups.com. So there is startups.com as another one of the featured companies today right along side of us. So I hit the link and my head started to spin. I had to look closely to see if I was just back in this site.
But of course, this site is WAY more awesome. They only have about 142 users compared to over 1,000 here plus a lot more activity.
It's using the same software.
That's the main problem with using off the shelf software rather than a more custom solution. (There are lots of benefits however)
This is a great example of how having the community split between two locations undermines the purpose of each site in the first place! OnStartups is rapidly becoming a great resource because of the huge number of users that quickly jump in with advice and tips.