I knew one person, and know of another, one from Canada and one from the US, about three years ago, who were denied re-entry because of the number of times they'd crossed doing the "visa run". I'm not saying that it's a problem, but if you ever have any bad luck, this would be where to watch for it. The person I knew was denied from a trip back from Uruguay when the officer checked his computer and saw she was red-flagged. She had been flagged previously and her case went to court and she hadn't known because the court didn't know how to get hold of her.
Being here without a visa is not illegal, it's just not "regularized". The more you put yourself in front of Immigrations, the more possibility you have of being flagged, maybe simply because someone was out too late partying the night before and was in a bad mood. The "run" doesn't really get you anything legally either, because according to the law you are over your stay anyway if you go over 180 days in a year. Remember, they are not issuing you a visa when they stamp your passport. It's a visa waiver, acknowledging that you entered the country. After your second "run" you've stayed longer than law allows, they're just letting you back into the country again any time afterwards on sufferance, per the discretion of the immigration official.
If you get flagged at the crossing, your case will go to an immigration court (assuming the paperwork doesn't get lost or something), where a judge will issue a letter to you stating that you have 30 days to regularize your status and report to him. If you don't get a precaria by then (first step in residency) or a visa of some sort through immigration channels (like a work visa), or apply for citizenship (I think), then after 30 days you WILL be illegal and could be deported. Of course, they're not going to come looking for you, but the next time you leave and come back in...
This is not intended to discourage anyone from making visa runs, or even leaving the country at will for whatever reason. I just wanted to clarify what my understanding of the law is so people could make an informed decision as to whether or not the "visa runs" were really a necessary thing to do, taken as that particular task and not an outing done for fun. I.e., take your trips when you want to, but maybe don't feel the need to do it right at the end of your "visa" period if you don't have to worry about a "visa". Bajo_cero has written about this previously in the mammoth immigration thread.
Of course, further disclaimer: The above is not intended as legal advice. You should consult with a licensed, trusted immigrations-specialist abogado before making any life-changing or disturbing decisions
