If by "?unknown?", you mean unprotected wireless hotspots you come across, then probably theres probably no risk as such. Even if there were some sort of malicious intention behind these hotspots, you are using a linux device and seeing as far the most malicious exploits are targetet at microsoft products you are propably safe.
However, if you are indeed talking about private unprotected hotspots, then you might be doing an illegal activity if you connect. Im pretty sure that in most countries unauthorized acces to hotspots is a crime along the lines of breaking and entering. How you would ever be discovered and punished is a completely different matter, so i guess its up to your moral beliefs