On my Ubuntu 11.04 64-bit system, this morning the Update Manager wanted to install the 2.6.38-10-generic kernel, so I let it. I haven't noticed any difference except that memory no longer seems to leak. Usually, after awhile the memory usage increases gradually and doesn't drop much, even when I close programs, but since installing the new kernel it seems to return to "normal" (about where it was after reboot) when I close programs. Of course, I don't really know if this is due to the kernel or to something else. Does someone out there know if this was a fix in the kernel?
I'm glad for the improvement, but don't really know what brought it about.
There are changelogs for the kernel packages. I recommend you read them, they usually detail the various changes to the packages/kernels, including reasons for the update to the package. (this answer was converted from a comment which I posted to this thread and removed after posting this answer)
After a day of using the new kernel (2.6.38-10.46) that Ubuntu's Software Update wanted to install on my system, I can say that there is no doubt that it has made a significant and very noticeable difference in memory usage. I ran everything I could so I could get memory usage up, and when I closed applications the memory usage just dropped back to normal.
In the past my system's memory use would slowly increase, even after applications had been closed. I have 4GB of RAM in my system, and so after a day or two it would seem that 1.5GB were tied up, then in another day or two it'd be up to 2GB, and it would keep going. Now, after closing applications I've been running, memory usage drops back down to 500 to 700 MB, which is around where memory usage is at right after I boot-up and before I've opened any applications. I've seen no signs that memory is going down a hole, or being held and not released after an application closes.