This report relates to rdesktop 1.6.0 (build 1.6.0-2ubuntu2).
My Ubuntu 9.10 (Karmic) 64 bit locked up suddenly (X stopped responding, sound looped, ctrl-alt-F1 didn't work).
Looking at the syslog I saw the following statement (that matches time-wise):
Jan 21 11:10:00 nsh-ubuntu kernel: [10308.902189] rdesktop: segfault at 1928dac ip 000000000041f637 sp 00007fff74e796b0 error 4 in rdesktop[400000+3a000]
I was working with rdesktop against a Windows Server 2008 Enterprise (6.0.6002 Service Pack 2 Build 6002) and writing a mail in Outlook 2007 (12.0.6514.5000).
I am not able to reproduce the problem.