HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main

you can control how many browser processes are used by IE 8:

The three most useful values for this setting are:

1.TabProcGrowth = – this is the default setting allowing IE 8 to control how many processes are used based on the amount of physical memory installed
2.TabProcGrowth = 0 – a value of zero means use the same process for all tabs, frames and pop-up windows in each browser session. This makes IE 8 behave just like IE 7 on Windows XP. There’s no frame merging and clicking on the IE icon creates a new instance of iexplore.exe in a separate browser session. Protected mode on Vista is not available with this setting because the IE frame and tabs can only operate at different integrity levels if they are in separate processes. This setting may be useful if you are debugging an IE add-on or if you have automated tests that rely on the IE 7 process model.
3.TabProcGrowth = 1 – this means use one process for all the tabs and pop-ups in a browser session and one process for the IE 8 frame window. Frame merging occurs with this setting and protected mode is supported on Vista. You may want to use TabProcGrowth = 1 where you want to keep tabs and pop-ups in one process but still allow the use of protected mode.
Specifying a number greater than 1 sets an upper limited on the maximum number of tab processes to use per frame window process.

A friend was describing the same issue to me and I found your suggestion and turns out he had the D addon. The key difference with the presence of this addon is the memory usage viewable via task manager. The new propagations of ie all had much much greater memory usage than the typical page.