My wifes iPhone 4 home button is quite tempremental these days, though mine bought at launch isnt.
Dont think hers is in warranty anymore, what are the odd's of apple doing anything about it?
When I had the same problem on my iPhone 4 it was three months out of warranty and they didn't want to know. Initially they did take it out the back and presumably cleaned it or some such, which made very little difference, I returned a couple of weeks later and just paid the £120 for a replacement.
I wish we had the two year warranty as they do in some eu countries.