Beta 11 indeed.I do most mining in my praire biome mining area but I have gone out of my way and went mining far away, with no luck.
I did some quick searching my own in a test world (creative w/ Tcon hammer) and dug a VERY long tunnel. I found some (very few) and never more in clusters larger than 3.
Diamonds are more common than lapis as far as I can tell from my main world, although they are sitll very uncommon.
(in my main world: found 3 diamonds so far and no lapis. To contrast it, I found 1 iridium ore already)
I did some quarrying, standard sizes in a test world, noted biome, and outcome of the most normal materials (for me at least). All quarries enchanted with Silk touch, so it's ore, not items:
https://docs.google.com/spreadsheet/ccc?key=0AqpehbCNum2kdHdDSWp4OTd2cHFIMjZRTlcxQVdnQ1E
Biome info taken from the F3 overlay.
I do agree, seems something is wrong with the lapis spawn.