openacs.org runs perfectly stable with tcl 8.5 and aolserver 4.5.1 since more than one year:
Database: PostgreSQL 8.3.5 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
Server: 4.5.1 (AOLserver)
Tcl: 8.5.6
XOTcl: 1.6.5
Tdom: 0.8.3
libthread: /usr/local/aolserver/lib/thread2.6.5/libthread2.6.5.so
Tcllib: /usr/local/tcl8.5.6/lib/tcllib1.10
acs-kernel: 2009-06-19, 5.5.1
xotcl-core: 2009-04-28, 0.106
xotcl-request-monitor: 2008-10-04, 0.42
xowiki: 2009-04-30, 0.118