No new firmware is planned for the Sony Ericsson P990 and W950 after 'R6D23', released two weeks ago. The last firmware release for the M600 was 'R6A16'. Explaining the termination, there's an official statement below from Niklas Sivander, Head of Product Management, Open OS.

"These firmware releases meet the requirements of bug fixes prioritized by our operator customers and the Customer Services organization within different markets. The decision to freeze the P990, W950 and M600 firmware versions at this point has been made based on meeting these requirements. As with any phone project, no product is ever completely flawless. Any remaining issues are of course unfortunate, but we feel that with the level of quality on the latest P990 firmware we now have a good, solid product.

Due to all heavy features in P990, the need for RAM memory has grown. When running multiple memory-consuming programs at the same time, the available RAM memory runs low and sometimes results in the memory manager closing down one of the applications. We have been working on improving memory management during the whole lifecycle of the P990. The low RAM memory situation for the P990 will unfortunately remain for the heaviest users, and would not be fixed by a new firmware update since it's hardware related. Multi-tasking on the P990 however works satisfactory provided that you don't run too large and memory-consuming programs at the same time. Performance improvements have indeed been made for the new P1 phone compared to P990 and the user RAM available for applications has been increased with >400%, significantly improving multi-tasking performance and application behavior so that you can run a lot heavier programs simultaneously.

Many users are comparing the P990 with the stable and relatively problem-free predecessor P910, which was an excellent smartphone. The P910 was based on an earlier Symbian OS and UIQ platform version with only an incremental software development update from the P900 (Symbian OS 7.0, UIQ 2.1), which made the development work a lot smoother at the time. With the P990, the Symbian OS capabilities have however expanded quite significantly and as with any major platform update, this was expected to introduce some new issues to solve. Sony Ericsson however underestimated the complexity of the development work for UIQ 3 and it took longer than expected to implement and quality assure things like the platform security mechanisms, the multimedia framework and other advanced connectivity aspects like 3G, videotelephony and WiFi. It has been a tough journey for all of us. Please extend my thanks and appreciation to the user and developer communities for all their patience, support and feedback which has helped us identify bugs and improve both existing and upcoming products."

Source: http://developer.sonyericsson.com/thread.jspa?threadID=39963&start=105&tstart=0