I was trying out the new Debian 6 Squeeze release on an old DL380 G2 and it complained about a missing piece of firmware (e100/d101m_ucode.bin). The missing code is a non-free binary-only module that you can install with firmware-linux-nonfree_0.28_all.deb. If you don’t install it then your network card won’t work.
To solve this issue, download the deb file and put it in the root of a thumbdrive. Plug the thumbdrive in the USB port at the back of your DL380 when prompted and Debian should find it if the deb-file is in the root of the drive (not buried in a folder somewhere). With this change I was able to get the network card going and it completed the DHCP set-up without further issues.
One way to avoid all this extra work is to install the OS using the installation disks that include the non-free binary modules. If you just want to get Debian running without the hassles of adding bits of non-free firmware then check out the installation media that includes the non-free firmware.