Apologies in advance if some of my comments are repetative, I'm repeating things in case Plinnell has not read them before.
Alt+click: select under (cycles thorough z-order at click point, try Inkscape CVS to see how it works)
note: this combination is widely used by windowmanagers (to drag the window). it was briefly discussed on the inkscape developer list but no better key combination was suggested.
Raise/Lower/Front/Back: we use PgUp/PgDn/Home/End, but since you are
I strongly advise against using Page Up and Page Down for anything other than going up and down the page, it bothers me that Inkscape (and so many other GTK applications) still do it. At the very least they should be combined with a modifier, but I still wouldn't recommend it, as they are awkward to reach keys.
If the keys are available I would recommend Forwards Ctrl+F Backwards Ctrl+B Back Ctrl+Shift+B Front Ctrl+Shift+F
(I'm assuming here that moving back or forward one is the more common action and giving it the easier key board shortcut than sending to the very back or very front)
Alteratively you could use these keybindings Forwards Ctrl+] Backwards Ctrl+[ and make Adobe Photoshop users happy.
It of course makes sense to look at what keybindings are used by Adobe InDesign or Quark Express because those are what your userbase are most likely to be familiar with.
Show grid/Snap to grid: we enable both with one key, since hardly anyone will need to see a grid without snapping to it. Our key is #, you could use e.g. Ctrl+#.
for many users that would be equivalent to Ctrl+Shift+3
== Tools
By tools I mean the buttons on the toolbar that starts with the arrow button. Due to the lack of tooltips, I will have to describe which tool I mean instead of using the correct term, for which you'll have to forgive me. These tools are likely to be switched often, so they need keys too (if they already have some keys I could not find them). Here are the keys Inkscape uses for approximately equivalent tools:
Inkscape inherited these keyboard shortcuts from Sodipodi, they are not necessarily good keybindings. For the Tools in Inkscape I think it would make sense to have single letter keybindings (right under your fingers, eassier to reach than Function keys), but depending on how your text mode is seperated this may not be entirely practical.
I dont know for sure but is possible the keyboard shortcuts were chosen to be similar to ones used by Corel Draw.
"Arrow": F1
This is particularly horrendous, definately goes against the Gnome Guidelines and probably goes against the KDE style guide as most applications use F1 for help.
Other tools might use the remaining F keys with modifiers.
Function keys are awkward enough without adding modifiers. It is impossible to have keybindings for everything so it might be necessary to reconsider any keybindings you have given to features that are not likely to be changed very often.