unless they get a warrant for the roll it up servers (could happen if they are located in united states, havent checked) then only roll it up admins can see your IP address or your isp could see you posting on to here, so they would have to get the info from your ISP or the rollitup servers themselves.
What does it mean if they did somehow get your IP Address? Well it could give them a reason for getting a warrant for search but even then its considered heresay I beleive, atleast I know it won't hold up in court unless you flat out admit that you made the post because nobody can prove it was actually you who made the post (someone could be using your internet or computer - if you have wireless even more likely to be the truth) - anyway I asked a lawyer about all this before and that is what he told me in short the only thing it can do is lead to a warrant for the cops to search your house but it can't be used as evidence against you once you get to court, and in order it to even get to the point of a warrant you are likely already under investigation and again rollitup would have to be raided before they could get your IP in the first place or your isp would have to release the information of what websites you have been visiting and than they would have to search thru all the websites you visit which likely they wouldnt because again this doesnt stand up as evidence in court.