Update: EA has noted that The Sims 3, due in June, will require Mac OS 10.5.7. At least we know it will be by then.
Several outlets are reporting today that Mac OS 10.5.7 is due to drop any time now. Build 9J47 was released on Friday, less than a week after 9J44, and build 9J50 is said to have been distributed on Tuesday. Until now, these updates have been delivered weekly. This accelerated distribution, along with a shortening list of fixes (the latest build only addresses three) are clues that 10.5.7 is nearly ready.
Additionally, some customers have reported that the ATI Radeon HD 4850-equipped iMacs that they ordered have had their ship dates bumped up to this week (there's a rumor that the graphics card requires 10.5.7).
We'll let you know when this shows up, and report any changes (or problems) we experience after installation.