Below is a list of links to sites and resources covering MIDI and Linux.
http://www.bright.net/~dlphilp/linuxsound/ 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
http://www.4front-tech.com/ OSS
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++. A great primer.