The following (possibly incoherent) ramblings were brought to you by my trip to Borders today.
If you have ever read a lot of Clamp manga, you'll notice that they seem to love making references to their other works in many of their stores. For instance, "X1999" is explicitly a semi-sequel to "Tokyo Babylon", "Magic Knights Rayearth" exists as a manga in the setting of "Angelic Layer", "Card Captor Sakura" has references to the game of "Angelic Layer", and Chobits also shares some past history and characters with "Angelic Layer". As you can see, the connections and implications can get quite complicated to sort through.
But with their newest (well, newest on the American market) two series', Clamp has finally sort of given an explanation for all this. You see, everything that Clamp writes exists in the same *multiverse*. Yes, multiverse, where there are also many, many alternate realities "based off" the series that take place in any given universe in that multiverse. And often, the events of one universe exists as a manga or whatever in another universe.
This is stated/implied in the first issues of "xxxHolic" and "Tsubasa" (which, incidentally, contain an explicit crossover between the two books in their first volumes - it's written on the covers).
In "Tsubasa", we see an alternate reality to the Card Captor Sakura universe where Sakura is a princess with some mysterious "Clow powers". We also see another alternate universe where Tomoyo (also from CCS) is a princess of a feudal Japan who has powerful transdimensional magics. We see yet another universe which seems to be a mixture of RGVeda and Chobits, where Chi (main character from Chobbits) is a catgirl instead of an android.
Individuals from those three worlds are sent to yet another dimension (which is the main setting of "xxxHolic"), to the shop of a "Dimension Witch", who gives them a guide named (and looks like) Mokona (of MKR - though it's explicitly stated not to be *that* Mokona) to take them on a quest through the Clamp multiverse. In the first world they travel to, they meet Sorata and Arashi (from X1999), though it's uncertain whether they're the Dragons of Heaven (Earth?) of their world.
Meanwhile, the Dimension Witch in "xxxHolic" makes casual references to Subaru Sumeragi (X1999) and "a girl who's master of the Clow" (CCS), who apparently also exist in that world. Though it's possible they're alternate versions of the CCS-series and the X1999-series.
So any time you see references of one Clamp work in another series, and it's not *explicitly* stated that the two series exist in the same universe, it can always be explained away by the alternate universe theory.
For instance, the invention of the Angelic Layer toys and the existence of certain AL people may appeared "onscreen" in Chobbits, but it's always possible that the specific storyline of AL didn't take place in the Chobbits world, but an alternate universe. Or, just because MKR is a manga in the AL-verse doesn't mean it also exists as an independent universe(s) where those events in the "manga" actually happened.
In other news, I also read a couple volumes of CCS today. It was kinda disjointed since they didn't have all the volumes (in order) there, but I think I got the gist of things. It's a pretty cool series. Though I didn't think much of the romance among the younger characters (they're in grade school, for goodness sakes. How can anyone seriously talk about romance and "one true love" when they haven't even hit puberty?)... but the more adult romance was kinda cool (hey, it has an explicit shounen-ai relationship between two hot guys ^_-).