The Norwegian tradition of using a sign called a house mark to distinguish your house from all the others dates back to at least 500 AD. As Hans Cappelen writes in the House Marks in Norway, people from all walks of life used them, and even cities and churches were known to have housemarks. They usually come in variants of a few basic forms, many of which incorporate letters from the futhark runic alphabet.
One of the basic forms often used is that of "the knot". According to Cappelen, the knot "consists of three or four loops made by curves, triangles or squares [and] is sometimes called 'St. Hans' cross' or the cross of St. John." The inamidst.com domain mark is derived from this device.
According to symbols.com, the Cross of St. John is certainly from the Viking period, and may have been used in Cabbalistic mysticism. It's also the place of interest sign (U+2318), which is called the sevärdhet in Swedish, and it appears as the command key on Apple keyboards. As Wikipedia says on its article about the command key, "The 'clover' symbol has no official name, but is often given nicknames like splat, butterfly, beanie, flower or shamrock." It's also sometimes called the "shield knot" or "earth square".
These days, we don't tend to have house marks, but we do have domain marks in the form of favicons and on-site branding. The mix of the ancient harking back to the original house marks, and the modern combining of "place of interest" with Unicode and Apple computers references, makes the Cross of St. John an obvious choice for a domain mark.
Sean B. Palmer