I was arrested during the WTO protests but never charged, so it hasn't been a big deal for me. As part of the class-action lawsuit any record of the arrest was supposed to be expunged, although I have heard from other WTO arrestees that they have had problems traveling internationally. I did have a WA state background check for a job a few years ago, and as far as I know it didn't show up.
Being arrested at a protest is almost always a misdemeanor, and usually if it's your first offense you can make a deal to get it taken off your record if you successfully complete probation for a year or two.
I know if you are planning on going to law school you don't want to get arrested, because of the ethical requirements lawyers have to obey the law. Likewise I could see it being a problem if you wanted to become a stock broker or something like that. A lot of employers are doing background checks now, and I could see a lot of protest arrests being used as a reason to not hire someone.
The main thing you would want to watch out for is to not fight against the police or do anything that could be considered assaulting an officer. That could be a felony, and you don't want to go there. Likewise leave the pot (or other illegal drugs) at home. Also don't get arrested in federal jurisdiction unless you decide to become a career protestor and want to do time in jail as part of that.
If you are thinking about risking arrest for Occupy Seattle, I would advise that you try to speak with some long-time protestors before you decide. For one thing a lot of activists will say they will support you, but then after you get arrested they don't show up. I saw this happen a few years ago with some people who got arrested protesting the Iraq War. I was lucky that there was an awesome legal support system set up for the WTO; I don't think Occupy Seattle has that.
I also think getting arrested should be strategic and more of a last resort. Yes it gets mainstream media attention, but we need to do more than get articles in the paper to enact economic change. Dealing with arrests can be very draining for a movement, and a lot of times grassroots organizing gets sacrificed for media attention. I really, really want a better economy, but I don't think getting arrested in Westlake is going to help that happen.
BTW I was there yesterday when people got arrested; anyone who thinks that was police brutality is a very privileged white person. Also comparing this to protests in Syria just belittles the bravery of the Syrians who went into the streets knowing they might get shot. Please keep some perspective, people. This is coming from an ally who plans to join Occupy Seattle protests when I can.