I worked for about three months on coding and two weeks on the concept. Yes, even though it's strongly inspired by Stackoverflow, it still requires some thinking about the implementation (especially the i18n stuff).
It was developed by @mitsuhiko and it has taken some work (you can check out the source code on http://bitbucket.org/plurk/solace/src/tip/solace/ - it's not a simple project). That said, it has not taken a lot of time, because mitsuhiko is a ninja ;)
Except that it isn't a reproduced product, and they largely won't be competing against each other. Stackexchange is offering a solution for it, Solace is just the software. They don't directly compete.
Now, I'd guess them being released on the same day isn't a coincidence, but I think its interesting to point out they don't directly compete.
amix here from Plurk. Our plan isn't really to compete against StackExchange. StackExchange seems to be targeted at a more enterprise crew and I am sure they will be successful at their market penetration (given the success of StackOverflow and FogBugz).
Our plan is to create a solution that we (our you) have 100% control over and that supports over 30 languages. Support is such a pain and Solace is mainly created to solve the issue surrounding support. Solace also offers an API that lets you integrate Solace deeply into your product.
Anyhow, the bottom line is that Solace is more targeted at startups and open-source projects - - while StackExchange is targeted at small business / enterprise market. That's at least how we look at it.
I'd say they do directly compete for some significant segment of the market -- anecdotally, I regularly compare services (or 'solutions') against installable products when researching the available options.
;-)