There are four buttons on my Wacom Bamboo tablet, how can I assign actions from keys.xml file to them? It would really help me if I could undo/redo with those buttons. I tried different settings in "File -> Input devices" dialog but it simply doesn't work no matter what I type there.

You can use your Wacom Tablet Properties to set the buttons to the keystroke you would like to use in Inkscape.

Where is "Wacom Tablet Properties" dialog? Is it only available with official drivers for Windows/Mac? Adding those two lines to .xinitrc has solved my problem:
xsetwacom set "Wacom Tablet" Button1 "core key ctrl z"
xsetwacom set "Wacom Tablet" Button3 "core key ctrl x"
But there is one more thing which makes me worry: no pressure sensitivity, I installed Wacom drivers properly (pressure works in Gimp and other software), I also checked on "Use pressure sensitivity" icon in caligraphy tool. All settings in "input devices" dialog are the same for Gimp and Inkscape. I have this problem on both Debian and Ubuntu, and on Inkscape 0.46 and recent SVN builds.

You haven't said but from the info you provided, I assume you're using Linux. I'm not a Linux user, so can't offer a lot of advise. I thought that pressure sensitivity was working in Inkscape under Linux. You might like to test if it's working ok in GIMP and if it's not you might need to ask about the problem on a Linux or Wacom/Linux forum--that's if someone on this forum doesn't already have the answer. If pressure is working in GIMP you might like to see if pressure is working in one of the Inkscape dev builds.

Oh, and don't forget that with the calligraphy tool there is a button on the tool control bar to toggle the use of pressure--make sure the button is down.