Just to update this thread, currently all is well with my L1. It warms up every morning now for the past week or two. I replaced the seals and greased the sleeve well, and don't have the late channeling issue anymore. In fact, shots are pulling better than ever before, so maybe it was just due for new seals. Fingers crossed it continues to warm up as it should.
Matt, can you update us on how your L1 is doing? Did the bleeding procedure from the nut behind the group work?

I'm glad to hear, Randy, that your machine has been behaving well. The bleeding procedure has not cured my L1's morning stall, although it has the same effect as a flush if done when I come into the kitchen after the machine has been on for an hour or more. My next move will be to pull the piston and do a visual examination of the new Londinium no-grease seals I put on sometime before the stall from cold began. I'll report back.

I promised to report back. I took my time about it, but I seem to have solved the problem by installing new piston seals. It's been about a week since I changed the seals and there has been no stall on starting up in the morning. On visual examination, the previous ones showed quite a bit of wear on the two seals closest to the shower screen. I suppose air was getting by them when the machine cooled down.

I have this issue, too. Piston seals were changed but not sleeve seals, in my case. Still have the problem. I've been long suspicious air is the culprit, but I have just made do, working around it with numerous flushes & extended steam wand releases. Now that I see it can be addressed once & for all, I'll have to get at the much bigger task & replace the inner sleeve seals. (Have been putting it off, as I'll need to get some odd parts & make a tool.)
I have yet to try the new greaseless seals, but read that the sleeve & group areas needs to be super-clean, using some sort of de-greaser & metal polisher; otherwise any hint of residue from grease can make them fail. I'm wondering what cleaner will do the job, whilst not being too abrasive at the same time? (The recommended Cif/Jif isn't available here.) BarKeeper's friend, maybe?? What did you use?

I used a citric acid solution as illustrated in this helpful [anonymized] video: http://www.youtube.com/watch?v=hCu2IBNgOh8

Ah, ok then, thanks! I misunderstood & thought we were to use a metal polish in addition to the citric soak. Phew!

I suppose if the citric acid doesn't fully do the job on the sleeve, a metal polish might finish it off.