If you're selling physical goods, then the answer is as soon as your devices are imported into the US. I once worked for a very small UK company which ran into

If you're selling physical goods, then the answer is as soon as your devices are imported into the US. I once worked for a very small UK company which ran into a patent tussle with a rather larger US competitor, as the former didn't want to exclude such a large market for potential sales. Evidence of prior art was collected, with the outcome being the overturning of their patent - at which point, a nominally amicable resolution was found. Software patents certainly pose challenges - even if the idea seems straightforward, if it's something fundamental to the design, you'll need to check for possible US patent conflicts if you want to enter the US market, or may in the future.