It was just one week ago that X Server 1.7.2
was released, but X Server 1.7.3 is now available today, which is far earlier than expected as it was not anticipated until the end of December. However, this X Server 1.7.3 release is out early because of two show-stopping issues that were found in the week-old 1.7.2 release.
X Server 1.7.3 corrects a break in the build system and the second bug is an ABI compatibility problem. X Server 1.7.4 will then end up being what 1.7.3 was supposed to be, about five weeks from now.
The X Server 1.7.3 release announcement can be read on the xorg mailing list