At the poorly-attended breakfast session at TechCrunch 50 (maybe because its 8am) - ComScore has announced quietly that they are releasing a self-service model "in the next few days". A publisher will be able to tag their own site and then collect standard ComScore reporting metrics.
This places them directly across the table from Quantcast and opens them to smaller publishers who traditionally cannot afford to work with ComScore. It is yet to be seen how ComScore will treat their self-service (supposedly free) model with their revenue-based model (which is prohibitively expensive for most).
Quantcast is the clear leader in the self service space. Compete is trying to differentiate themselves through premium data. To date, ComScore has served the large publishers... interesting to see what traction ComScore gets with the smaller publishers... there is only room for so many tracking pixels on each webpage.
Also interesting to note that ComScore has now mentioned Quantcast in passing a handful of times during their presentation.