I get the message when I am moving bookmarks in the "Web" app.
I am trying to organize my imported bookmarks and I can only move a few at a time before I start getting the "low memory" message.
(I do need to clean out my bookmarks -- it is basically the same set that I have moved from the 770 > n800 > n810 and now to the n900).

I haven't taken the time yet to actually try to figure out what is happening i.e using "top", etc.