BEA only released EJB 3 Support about a month ago.
JBoss did it a while ago but their EJB GA version is no more than 3 monnths old i think (the version that they say the recommend for production use).
IBM Websphere 6.1 with EJB? Only several months old.
If youre starting a 2 year (calendar time) project, go ahead, the technolcogy will be ok when the time for production comes but if your starting a tight 6 month project today, I recommend doing it the hibernate way,
I had (and still having) my own share of scary bleeding edge near-GA
java products and i prefer to take it more slowly this time,