As reported at TechCrunch, the popular Twitter client twhirl created by German developer Marco Kaiser has been acquired by video chat service Seesmic. Besides the Twitter API, it also uses the Pownce API and the Jaiku API to allow cross-posting messages to those services. It is a very popular and very useful client that will have an interesting future. For more background on the acquisition, Seemic's Loic LeMur gives 20 reasons why they acquired it.
On a related note, acquisitions of mashups is becoming a mini-trend that we'll report on in more detail shortly.