Below is a list of links to sites and resources covering MIDI and Linux.
http://www.linux-sound.org/ Sound & MIDI Software For Linux. A huge resource of MIDI and Audio related information.
http://www.linuxdj.com/audio/lad/ Linux Audio Developers mailing list. For developer related discussion.
http://www.linuxdj.com/audio/quality The Linux Audio Quality HOWTO. A good section on MIDI cards, essential reading for building a Linux studio.
http://www.alsa-project.org/ Alsa Project - Audio/MIDI Driver
http://www.4front-tech.com/ OSS - Audio/MIDI Driver
http://www.gardena.net/benno/linux/audio/ Low Latancy patches
http://ccrma-www.stanford.edu/~craig/articles/linuxmidi/ Introduction to MIDI programming in Linux. Craig Stuart Sapp covers writing basic MIDI utilities in C/C++ for the OSS driver.
http://www.suse.de/~mana Excellent collection of ALSA 0.9 MIDI and PCM example C programs by Dr. Matthias Nagorni.