Just used a starter for the first time myself a few days ago. Made a 900ml starter for WLP023 and put it on a stirplate for just shy of 48 hours.
Since I used a stirplate, my starter was a tad foamy, but I let it sit for a few hours before pitching, and the yeast was clearly starting to fall to the bottom. I agree with opteek, you have CO2 and a thin layer of krausen beginning to form. Just stir that bad boy every time you walk by and watch out during your fermentation... see below.
Made beer on Wednesday afternoon, around 3pm, this is how the fermentation is going 37 hours later... For the record, I've NEVER had such an aggressive fermentation. 6.5 gallon better bottle, 5.25 gallons of wort, first blow off tube ever needed by me, jestmaty