It was announced today at the CLAMP FESTIVAL 2012 NAGOYA that a new series of the XXXHOLiC manga will begin in February.
The new series will be called XXXHOLiC ◆ Rei (XXXHOLiC ◆ 戻 < レイ >) and will be published in Young Magazine. The kanji for 戻 roughly means "return".
This will coincide with the release of the XXXHOLiC artbook, the TV Drama and also the series' 10th Anniversary.
All of this makes a lot of sense, if you ask me. There couldn't be a better time, strategic-wise, to resume the series. I would presume that Kodansha and CLAMP already knew that there would be a live action adaptation and decided to put the series on hiatus in order to wait for it. I'd say that everything was planned from the start.