According to AnimeNewsNetwork:
This year’s 25th issue of Kodansha‘s Weekly Young Magazine announced on Monday that CLAMP‘s xxxHOLiC: Rei manga will reach its “climax” in next week’s issue on May 30. (Note: In Japanese, the word “climax” written in the phonetic katakana alphabet often refers to the final chapter of a manga.)
This means that the next chapter is last one. Update: Chapter 51 is out and it’s not the last one.
Considering CLAMP’s history of bringing back XXXHOLiC, this might not be the last we’ve seen of it, but it’ll be going off for a while at least.
On another news, the release date of Tsubasa – WoRLD CHRoNiCLE – #3 has been pushed back to August 17th.