Dan89, realize this is 20+ year-old information (just saying that puts me into a special kind of state of depression ), but when I went through ROTC (yeeears back! Yep, still depressing to say that) there were a few cadets who did both the ROTC program and joined a Fraternity. So, it can be done.
Now, a word of caution. ROTC, while not as time-consuming as the daily grind of a SA, still can be a big drain on your college time-managment requirements if you want the experience to be successful. You can be the kind of cadet who just shows up one day a week for drill and inspection and nothing else for your first two years in, but who do you think the Detachment Commander is going to give the best recommendations to when it comes time to hand out career assignments? The guy who puts in the extra effort and is showing commitment and leadership, or th eguy skating by? Oh, and BTW, you WILL be busy your Junior and Senior year with leadership positions in the unit.
It comes down to priorities. If you want both the ROTC experience and the Greek experience, it can be done with just a little time-management effort on your part. You're the one who needs to look inside yourself and ask the reasons why you want to do both.
Fraternities are great social networks (I don't buy the line anymore that they are there for "charitable service" as well as the social part; IMHO, that has ceased to be a primary factor for anyone interested in joining since the 70s). As a social network, you'll make life-long friends, and can use those connections post-graduation in numerous ways. (It's not just the image most people have now of a filthy huge house filled with guys sleeping off last night's party, but that does occur). If that is what you desire, you need to go through RUSH week and see what each has to offer.
But ROTC also offers a chance for you to find other individuals who share common interests with you as well. There are organizations to join in ROTC (The Arnold Air Society for AFROTC comes to mind; National organization at most AFROTC Dets), cliques of people to form you own social networks around, and lots of other ways to get involved (but I doubt you'll walk into the Detachment building on a Monday and find it a mess with beer cans stacked up in the corner like you would in a Frat).
Bottom Line: the choice to spend the effort on both is up to you. YOU have to determine if it's worth it based on your priorities. My final advice: look at what you want past college. If it's a commision and a career in the military in your dream job, concentrate on ROTC to garauntee you get that path. If it's an opportunity to make life long friends and a network that can get you into the civilian job market through contacts (plus some "fuzzy" memories of weekends in college) consider joining a Frat.