Hi everyone,
The problem with accented characters appearing at the end of the line is (almost) FIXED. You can get Tk 8.5 or 8.6 CVS, or you can find the patches here:
https://sourceforge.net/tracker/?func=detail&atid=112997&aid=1924761&group_id=12997and new packages for Ubuntu Karmic and Lucid here:
http://amsn-project.net/~vivia/(8.5.7 are for Karmic and 8.5.8 are for Lucid)
About the remaining issue:
Affected users are those who have BOTH deadkeys AND XIM (or SCIM/UIM/ibus, they all use XIM for aMSN).
In this case, characters typed using deadkeys will appear normally only if Tk isn't busy. If Tk is busy, instead of getting delayed, they might disappear completely. In a few words, this means: If you have cam on, double-check to make sure your accented characters are in place. I know, it's still much better than all your letters getting misplaced

EDIT: I just tested it on a real use-case scenario while using webcam, and all my accented letters appeared. Seems like the problem appears only when aMSN is frozen for several seconds.
I will let you know if there are further news.
EDIT2: Packages for Ubuntu Lucid added