Wow, it's even worse than I realized. Even when I have closed
the two applications that start automatically now (browser opens
to a previously but not now set home page, and blank new
document of Open Office Word), those same two applications
continue to open upon boot, every time. It took a few
iterations of that before I noticed.

Tried re-checking the box, rebooting, then unchecking it
again and rebooting. Same problem. Somehow the same two
applications that happened to have been open when I shut down
that first time I checked the box have been frozen into my
startup routine.

If this may be of note: I loaded Startup-Manager and tested
using it to tweak my grub menu, instead of doing it manually
by editing boot/grub/menu.lst as is my habit. (FWIW to
someone: I was able to change the user timeout delay therein
under "Boot options" but unable to "Limit the number of
kernels in the boot menu" under "Advanced"). Might having
loaded Startup-Manager had something to do with this? It
doesn't seem likely, as this seems to have to do only with
the bootloader, not how Gnome loads up. I'm pretty sure
this is a problem in Gnome and Sessions.

What sent me to System --> Administration --> Sessions
there in the first place? To find out what might have
been causing the shutdown window to take so long to appear,
as per another recent thread. I did find, BTW, that by
disabling one of the Startup Programs in the Sessions
Preferences window there (the Italc client daemon) that
the shutdown problem is gone. (Thanks for the advice
on that.) I don't see why this would have anything
to do with my current startup sessions problem, but
thought I'd mention it.

So, aside from the GUI provided by Sessions Preferences,
is there another way to deal with what applications
start up under Gnome?