I'd like to add that I have built a step sequencer based on the Arduino and the MIDI library and am currently facing a problem with sending multiple notes-on. Basically, I figured that I could set it up so that at any given step, I could send all my note-on instructions based on whatever my grid was set up as. And then when it was time to move to the next step, send all my note-offs for the previous step, then send my note-on messages for the current step ("next step" relative to the previous one). But what I am finding, and I don't know if this is just how MIDI is or if this is something in the MIDI library....but what I am finding is that when I send a "note-on" message, then I send another "note-on" message before sending a "note-off" message for the previous note, the MIDI messages are glitching and it is not operating correctly. My fix for this, at the moment, is to only ever have only "one note on at any given moment". I haven't yet implemented my idea. I'm going to write something up and test it out. But I wanted to make you aware of this issue that I'm having so if you were thinking of something that is going to be playing. But, you know, maybe that is just how MIDI works, I still have a lot to learn about it and it's totally possible that I'm just doing it wrong but I wanted to at least add my $.02.
Also, since I found this thread through a google search, I hope this helps anyone else who is suffering the same problem.