Not being in Beta, I can't say for sure how open PVP is going to work. Warhammer had some elements that created a great start for open warfare.
Additionally, it seems that the main story has somewhere over 200 hours of playtime. A quick search shows about 15 - 25 hours a week of average play time for the average player. At 20 hours... that's 2 and a half months of time for SWTOR to REALLY implement a rated system before the bulk of people are going to cry foul... realistically 3.5 months before people start to cry. When I say people... it's the masses (those not going nuts on forums)... that they should be basing their decisions on... not the extreme population.
These are rough figures that I'm sure plenty of you will take the time to show are wrong, but my point is... we aren't going to all jump in ready for level 50 competitive pvp on day one. All of us are going to play, learn, evolve, and adapt as we progress through the game. Bioware is going to evaluate how we interact with the game, and further evolve a system i am confident is already somewhat in the works, even if just in blueprint form. Six months may be a bit much, but 3 months isn't unreasonable. By 6 months, I'm hoping for 3 more flashpoints, 2 more operations, another warzone, and maybe a fun end game non-combat activity. I doubt I'll get all of this, but I know Bioware isn't a company who is going to sit back and hope things just work.