I just pitched a 1/4 of a yeast cake (from a 2.5 gallon batch) into a 1 pint starter (1.040) inside a 1L bottle. I did this last night at 10 PM and had krausen out of the airlock this morning by 7 AM. I washed it off, re-sanitized the airlock...and it was filling up again within a hour! I left it inside a bucket to minimize the mess I'll have when I get home... Should make for an interesting brewing session on Friday night!
Oh yeah, krausen definately will form even on a stirplate. Had a 1000ml flask going with a starter of WLP001. Although this isn't by any means a very explosive yeast, I still had krausen coming out the top. I just use a sanitized piece of foil instead of an airlock for the starter. Much easier to clean and replace. I just cleaned up the stuff that came out of the starter and made sure that the foil was still secure around the top of the flask.