Just thought I would check this with everyone, but there appears to be a memory leak when joining meshes. I was trying to make a bunch of raindrops into a single object, and the memory use kept increasing every time I would join the meshes. (it went up a lot)
It could be to do with the Undo levels that are being created whenever an operation is done in blender. it would effectively double the memory usage of those objects each time (AFAIK)
Your job is to report it to the bug tracker.
Give the circumstances and the hardware/OS that you use.
Join a .blend example file and a clear description of every step needed to replicate the bug.
You should do that even here as I am not able to cause the bug to show right now with the indications that you give: Ubuntu 7.04 32bit
I was joining hundreds of 32*32 UVshperes. With stuff less intense than that it may be less obvious