Before showing the numbers I will repeat again that I think the IFs of the journal where a paper is published is a very poor measure of a papers importance. Although it is probably a good measure of the relative value of a journal (within a given field) we should be striving to pick what we read based on the value of a paper instead of the journal.
The Impact Factors that will be published this year are calculated as the total number of citations from 2008 to papers published in 2006 and 2007, divided by the number of citable units in 2006-2007 (articles and reviews). The data I am looking at is from Scopus so it varies a bit from the one in ISI. The variability comes from the decision of what to include as "citable" articles and from the journals that are covered in Scopus versus ISI.
One problem I found with Scopus data was that, for some journals, the database has multiple entries due to small variations in article titles. For PLoS Biology, PLoS Computational Biology and PLoS Genetics the number of articles published should be less than half of what is reported. This does not appear to be the case for PLoS ONE.
I downloaded the tables of published articles and tried to removed redundancies looking at the tittles and authors. I counted only articles and reviews as citable items but used all articles published in 2006-2007 to get the number of citations in the year 2008. I also did the same calculations for the impact factor of the previous year to be able to compare with the data from ISI. The results were comparable but not the same.
In summary, PLoS ONE might get an impact factor of about half of the expected for PLoS Computational Biology. The usual disclaimers should be said: I have no idea of how complete Scopus data is and how exactly it relates to ISI.
Update:
The official impact factor for PLoS ONE for 2008 is out and I is ~ 4.3. I underestimated it by 1.5. It is also amazing how many people search for this online. This post is my number one source of traffic to this blog. If you are reading this and typically sit in panels that decide on new faculty, please stop evaluating people by where they publish. This way, postdocs like me can focus on doing interesting science instead of trying to get into nature/cell/science.