If you configure environment variables in any slave (using nodes>"slave_name">Configure), they aren't set in the slave.
Example:
I am using selenium grid and I need to set DISPLAY=:20 in one linux slave (to use selenium:xvfb). When starting that slave, my log is:
[01/15/10 10:09:05] [SSH] Authenticating as hudson/******.
[01/15/10 10:09:06] [SSH] Authentication successful.
[01/15/10 10:09:06] [SSH] The remote users environment is:
BASH=/bin/bash
BASH_ARGC=()
BASH_ARGV=()
BASH_EXECUTION_STRING=set
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="3" [1]="2" [2]="48" [3]="1" [4]="release" [5]="i486-pc-linux-gnu")
BASH_VERSION='3.2.48(1)-release'
DIRSTACK=()
EUID=1006
GROUPS=()
HOME=/home/hudson
HOSTNAME=BMV-0116
HOSTTYPE=i486
IFS=$' \t\n'
LANG=es_ES.UTF-8
LOGNAME=hudson
MACHTYPE=i486-pc-linux-gnu
MAIL=/var/mail/hudson
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
PIPESTATUS=([0]="0")
PPID=15239
PS4='+ '
PWD=/home/hudson
SHELL=/bin/bash
SHELLOPTS=braceexpand:hashall:interactive-comments
SHLVL=1
SSH_CLIENT='192.168.5.110 44879 22'
SSH_CONNECTION='192.168.5.110 44879 192.168.5.107 22'
TERM=dumb
UID=1006
USER=hudson
XDG_SESSION_COOKIE=f635da413ef2204c774938274adc4480-1263546546.174106-1094123142
_=']'
[01/15/10 10:09:07] [SSH] Checking java version of java
[01/15/10 10:09:07] [SSH] java -version returned 1.6.0_16.
[01/15/10 10:09:07] [SSH] Starting sftp client.
[01/15/10 10:09:07] [SSH] Copying latest slave.jar...
[01/15/10 10:09:08] [SSH] Copied 211.022 bytes.
[01/15/10 10:09:08] [SSH] Starting slave process: cd '/home/hudson' && java -jar slave.jar

As you can see DISPLAY=:20 is not exported, and my selenium tests fail (
"Display not set")