Building dependency tree... Done
Calculating upgrade... Done
The following packages will be REMOVED:
apt apt-utils aptitude base-config cmake dselect fam gcc gcc-4.1 gettext-base groff-base
libfam0 libsasl2 libsigc++-1.2-5c102 libsigc++-2.0-0c2a libssp0 libstdc++6 lighttpd
man-db nmap python-apt tasksel tasksel-data telnet
The following NEW packages will be installed:
bind9utils libbind9-40 libcap2 libclamav5 libcurl3-gnutls libdb4.5 libdns45 libfsplib0
libgpm2 libisc45 libisccc40 libisccfg40 liblwres40 libpq5 libruby1.8 libtimedate-perl
libxcb-xlib0 libxcb1 lsb-release lynx-cur openssl-blacklist python2.5 python2.5-minimal
ttf-dejavu-core ttf-dejavu-extra
The following packages have been kept back:
clamav-freshclam cpp-4.1 dpkg dpkg-dev elinks gcc-4.1-base libcurses-perl libgcc1
liblocale-gettext-perl libperl5.8 libterm-readkey-perl libtext-charwidth-perl
libtext-iconv-perl perl perl-base perl-modules python-support
The following packages will be upgraded:
bind9 dovecot-common dovecot-imapd dovecot-pop3d libsasl2-2 libx11-6 linux-libc-dev lynx
pdmenu python python-minimal python2.4 python2.4-minimal ssl-cert ttf-dejavu
WARNING: The following essential packages will be removed.
This should NOT be done unless you know exactly what you are doing!
apt libstdc++6 (due to apt)
15 upgraded, 25 newly installed, 24 to remove and 17 not upgraded.
Need to get 30.2MB of archives.
After unpacking 836kB of additional disk space will be used.
You are about to do something potentially harmful.
To continue type in the phrase 'Yes, do as I say!'

cat /etc/debian_version
5.0.1

deb http://ftp.us.debian.org/debian/ stable main
deb-src http://ftp.us.debian.org/debian/ stable main

deb http://security.debian.org/ stable/updates main

deb http://security.debian.org/ etch/updates main contrib

Have you tried aptitude instead? ...

Edit: also, make your sources.list consistent - either include contrib in all lines or not at all

Are you upgrading from etch to lenny? ie, why are you doing a dist-upgrade?

The following packages are BROKEN:
gcc-4.3 libgcc1 libgomp1 libperl5.8 libsasl2 libssp0 libstdc++6
The following NEW packages will be automatically installed:
bind9utils elinks-data libbind9-40 libc6-dev libcap2 libclamav5 libcurl3-gnutls
libcwidget3 libdb4.5 libdns45 libept0 libfsplib0 libgpm2 libisc45 libisccc40
libisccfg40 liblwres40 libperl5.10 libpq5 libruby1.8 libtimedate-perl libxapian15
libxcb-xlib0 libxcb1 libxmlrpc-c3 lsb-release lynx-cur lzma openssl-blacklist
python2.5 python2.5-minimal ttf-dejavu-core ttf-dejavu-extra
The following packages will be automatically REMOVED:
libclamav2
The following NEW packages will be installed:
bind9utils elinks-data libbind9-40 libc6-dev libcap2 libclamav5 libcurl3-gnutls
libcwidget3 libdb4.5 libdns45 libept0 libfsplib0 libgpm2 libisc45 libisccc40
libisccfg40 liblwres40 libperl5.10 libpq5 libruby1.8 libtimedate-perl libxapian15
libxcb-xlib0 libxcb1 libxmlrpc-c3 lsb-release lynx-cur lzma openssl-blacklist
python2.5 python2.5-minimal ttf-dejavu-core ttf-dejavu-extra
The following packages will be REMOVED:
libclamav2
The following packages will be upgraded:
apt apt-utils aptitude bind9 clamav-freshclam cmake cpp-4.1 dovecot-common
dovecot-imapd dovecot-pop3d dpkg dpkg-dev elinks gcc gcc-4.1 gcc-4.1-base
libcurses-perl liblocale-gettext-perl libsasl2-2 libterm-readkey-perl
libtext-charwidth-perl libtext-iconv-perl libx11-6 lynx nmap pdmenu perl perl-base
perl-modules python python-apt python-minimal python-support python2.4
python2.4-minimal ssl-cert ttf-dejavu
The following packages are RECOMMENDED but will NOT be installed:
aptitude-doc-cs aptitude-doc-en aptitude-doc-fi aptitude-doc-fr aptitude-doc-ja
build-essential libparse-debianchangelog-perl libsasl2-modules
40 packages upgraded, 35 newly installed, 1 to remove and 0 not upgraded.
Need to get 69.0MB of archives. After unpacking 78.0MB will be used.
The following packages have unmet dependencies:
libstdc++6: Depends: gcc-4.3-base (= 4.3.2-1.1) but 4.3.3-3 is installed.
libsasl2: Depends: libsasl2-2 (= 2.1.22.dfsg1-8) but 2.1.22.dfsg1-23+lenny1 is to be installed.
libperl5.8: Depends: perl-base (= 5.8.8-11.1+lenny1) but 5.10.0-19 is to be installed.
libgomp1: Depends: gcc-4.3-base (= 4.3.2-1.1) but 4.3.3-3 is installed.
gcc-4.3: Depends: gcc-4.3-base (= 4.3.2-1.1) but 4.3.3-3 is installed.
Depends: cpp-4.3 (= 4.3.2-1.1) but 4.3.3-3 is installed.
libssp0: Depends: gcc-4.1-base (= 4.1.1-21) but 4.1.2-25 is to be installed.
libgcc1: Depends: gcc-4.3-base (= 4.3.2-1.1) but 4.3.3-3 is installed.
Resolving dependencies...
The following actions will resolve these dependencies:

Remove the following packages:
libperl5.8

Keep the following packages at their current version:
cpp-4.1 [4.1.1-21 (now)]
gcc-4.1 [4.1.1-21 (now)]
gcc-4.1-base [4.1.1-21 (now)]
libsasl2-2 [2.1.22.dfsg1-8 (now)]

Downgrade the following packages:
cpp-4.3 [4.3.3-3 (now) -> 4.3.2-1.1 (stable)]
gcc-4.3-base [4.3.3-3 (now) -> 4.3.2-1.1 (stable)]

Leave the following dependencies unresolved:
dpkg-dev recommends build-essential
Score is -874

http://www.debian.org/releases/lenny/i386/release-notes/ch-upgrading.html

always good to read the documentation, no?

How did you get packages from Squeeze? cpp-4.3.3-3 is not in lenny but was in Squeeze as of 02/16.

One of the first things noted in the release notes is that you should upgrade apt first. Perhaps the first thing you should do now is apt-get install apt aptitude .