FYI, there are actually two attempts to produce a culture-spreading mechanism. Both make use of traderoutes, I believe:
Mylon:
http://forums.civfanatics.com/showthread.php?t=145005
The_Lopez:
http://forums.civfanatics.com/showthread.php?t=170884
I think, however, this is only part of the battle. To quote from an earlier post, there are really three dimensions to this:
1.
(DONE!) Culture is Transmitted by Trade Routes. "+3 Gold from Athens" also means "+3 Culture from Athens", or perhaps some kind of ratio.
2.
(Incomplete) Culture is generated proportionally according to % nationality. So if a city generates 5 culture per turn, but has 40% foreign nationality, then 3 culture per turn will be domestic and 2 culture per turn will be foreign.
3.
(Incomplete) Culture degenerates into 'local culture' according to maintainance costs. Not sure what the ratio would be, but at 1:1 "-2 Maintainance" would mean "2 Culture per turn becomes 'city' culture".
Overall:
(Incomplete) Now that culture spreads, what will it do? Now that culture isn't just generated by a city but flows from other cities and other nations, cities will get culture very quickly, but not necessarily your culture.
I think 2 and 3 are very important -- for culture to erode, and for foreign culture to naturally propogate in foreign cities. As well, we have to ensure that #1 is done properly, that culture flows in ways that make sense.