SEMC Developer Support posted the following message:
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Hi all,
Sorry for the delay in getting this info. No new firmware is planned to be released for the P990 and W950 after R6D23 released on June 15, 2007 via the Sony Ericsson Update Service. The last firmware release for the M600 was R6A16.
I have got an official statement from Nicklas Sivander who is the Head of Product Management, Open OS.
So here it goes:

"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."

In addition to Niklas's comment above, I have managed to get hold of a list of the major bug fixes as required by operators and Customer Services and solved as of R6D23, I need though to translate some of the stuff into a non tech jibberis before I publish them.

Sony Ericsson has not found any memory leakage problem in the platform or product in its testing of the latest firmware. Those additional third party applications that users may download should be ok and not create memory leakage provided that they have been developed according to UIQ 3 standard guidelines but it is up to the developer in question to ensure correct development and testing procedures of their apps.

The comments about new bugs from our forums and http://uiq3.sourceforge.net/w[....]ex.php/Phones/P990i#Known_Bugs having been introduced in R6D23 have been forwarded to the development projects and will be reviewed over the coming days and I will try and get some comments back to the forums from product development on them.

Bugs reported on this forum and from the P990 wiki at sourceforge has been sent to the development organization. Many of the reported bugs have been fixed as of R6D23.

I appreciate your continued patience on this topic and fully understand your frustration and disappointment on our lack of communication and clarifications up until now.

Cheers
Johan