I haven't seen this posted here before, but then I haven't been too regular about visiting lately. In case you didn't know, I just pulled this from our own US Dept. of State website, Travel Warnings page ....
http://travel.state.gov/ukraine.htmlHere the text, copied and pasted ....
Effective July 1, 2002 the Ukrainian Government introduced visa free travel for stays of up to eight days for American citizens arriving at the international airports of Simferopol and Odessa and at the international port of Odessa. Travel must originate abroad and end at these ports of entry without any other initial stop in Ukraine. Travelers must also depart Ukraine from the same port used at the time of arrival.