Just found a small bug - open a session with say 5 or 6 tabs then add a few more tabs that you open through bookmarks or directly through the URL bar THEN right-click on the Reload button and choose Reload all tabs : only the tabs that were opened through the session will be reloaded.
No big deal but still...
I too use KM 1.5.4 and WinXP SP3 and experience the same problem as mhf but I did not think it was a "problem" and I simply re-save the session with the same name.