A little over a week since the last beta
, Apple is seeding a new build of OS X 10.8.4 to developers. The release is listed as 12E55, and has no known issues. People are asked to concentrate testing on Safari, Wi-Fi, graphics, and Windows filesharing.
The pace of v10.8.4's development has so far been intense, with new betas coming roughly once per week. Apple may be in a hurry to finish the code ahead of WWDC on June 10th, since the company should be releasing the first beta of OS X 10.9 there, and may also need to have v10.8.4 ready for updated Macs.