Randomly selected page from Woot designs (happened to be #6, generated by a random number generator) yielded approximately 2 homophonic designs (approximately, because the second one involved owlgebra so it doesn't REALLY count). 2 out 40= 5%.
Assuming woot is about average as far as such things go, 5% of all graphic shirts on average, deal with homophones.
According to http://wiki.answers.com/Q/How_many_shirts_does_the_average_American_own#slide3 average American has 35 shirts, with 5% of the 35 - homphonic =1.75, to the total of 525 mln hono shirts in the US.
Wikipedia points to 1913 as one of the alleged years of origination of t- shirts, to the total of a 100 years.
Assuming 0 in the beginning; growth rate to about 2 shirts on average by about 1953 (population about 160 mln), and growth to 35 shirts by 2013; assuming average shirt lasts 2 years and needs to be replaced every 2 years.
Actually, about mid-50s is when graphic t-shirts started anyways; so lets start with 1953 instead of 1913.
Assume the US owns half of world's t-shirts. (just based on stats of consumption of other things).