We use tweetmeme for the heck of it and we like the service. But today I read on Techcrunch on the war that has been going on for a while.
Similarities between the sites are somewhat close but so are the thousands of Pligg (an open source DIGG like CMS) based sites. Most probably both these tweet sites got their ideas from similar sites, DIGG Pligg.
We too have a Pligg based site coming up and the looks are not much different. The Pligg existed even before Twitter itself so were the designs. So laying right to design will be a bit off the scale. According to TC the creators of retweet.com has been hovering in the twitterati sphere with many other twitter based products that seem to be doubvious, like hummingbird. You can read all about them at TC.
The way twitter has been trademarking terms, there might be another war! Hope Google will not try to trademark "search"!