While a minor issue, tweets from third party clients help to advertise their use. For example, it's a bit disconcerting to check the official TweetDeck account, and see they are updating "from Web", ostensibly not using their own product.
Users Clamoring About Twitter's Missing Info
I first noticed the bug on my own feed, as it said that items I had redirected from FriendFeed, using the FriendFeed URL shortener, ff.im, had originated from the Web. I also couldn't see any use of the desktop application Tweetie, when I know I have been using it. Surprisingly, Twitter showed "from Web" as the only service I had used at all.
This Tweet Originated on FriendFeed, not on Twitter's Web site
TweetDeck Users On Twitter Asking About the Bug
And yes, in a world of 140 characters, even the little things get noticed. Though not the bonfire of recent missteps, this issue has seen many asking over the last five hours why their clients aren't accurately being displayed. (See: Twitter Search "from TweetDeck" for example)
I don't see this being a longtime bug. Twitter will likely figure out the issue, which might have been introduced as a result of their recovering from unplanned downtime on Saturday. But for diehard Twitter client fans, and believe me, they're out there, the disconnect is yet another head-scratcher from a service that's playing a bigger role for an increasing number of people.