AOL and pollster John Zogby (best known for his election surveys) teamed up for a national study of 20-69 year-olds' views of love and relationships. Summaries of the findings are available [this link updated 10/3/08].
Of greatest relevance to Emerging Adulthood, of course, are the results for the youngest respondents in the survey. Several age-group differences are discussed in the aforementioned results document, but they appear fairly modest in magnitude. For example:
More than 4 in 10 Americans (44%), including 50% in their 20s don't believe that they need to be married to validate the commitment of a long-term relationship...
As I've often expressed in the past, studying the dynamics of young adults' attitudes, decision-making, and relationship establishment in the romantic domain would seem to be a fruitful research area.