This is a tracking bug to use IBus native function 
'ibus_engine_delete_surrounding_text' on IBus 1.2.1 and later instead of always 
forwarding backspace key sequences to delete the characters placing before the 
cursor position regardless of IBus version.

When we implemented nested calculation in ibus-mozc at r47, we decided not to 
use 'ibus_engine_delete_surrounding_text' because it was not functional on many 
applications.
http://code.google.com/p/chromium/issues/detail?id=55685
http://code.google.com/p/mozc/source/browse/trunk/src/unix/ibus/mozc_engine.cc?r
=47#525

Now, as far as we have tested on Ubuntu 11.10, 
'ibus_engine_delete_surrounding_text' seems to be functional and stable on the 
major applications such as Firefox 10.0, OpenOffice 3.2, and GEdit 2.30.

So it seems to be safe to start using 'ibus_engine_delete_surrounding_text'.