功夫 and 工夫 are frequently used homophones - the pronunciations are the same - but the meanings have differences, so it is necessary to clearly distinguish the meanings and differentiate the usages.
工夫 has three meanings: 1. A period of time that is occupied; for instance: after a moment of gongfu the task is completed; using two years of gongfu writing, a single book is written; 2. A period of free time or leisure time, for instance: right now, I don't have gongfu; on weekends, everybody has gongfu, otherwise they travel; 3. A moment [when something happens], for instance: although I am done with that gongfu, I am still but a child.
功夫 primarily points to a person's skills, achievements, etc. For instance: his acting truly had gongfu, the actors all achieved practiced gongfu.
The difference between the two words lies in this: 工夫 indicates time; 功夫 does not indicate time but refers to a person's skill.
the_economist wrote:The phrase 工夫 often refers to labor, as is pointed out by the Language Log post, and interestingly, also refers to leisure time (see http://www.5dhz.com/hanziliushu/shuowenjiezi/2009-09-04/1088.html --- in Chinese). I interpret 工夫茶 to therefore encompass elements of labor and skill, as mentioned by Will and Poohblah, and also a leisurely time. It might not be a long time, indeed it isn't generally a very time consuming process to brew 工夫茶.