Some Muslims would be offended, some wouldn't. As Americans have modified the ways we spell Peking, Madras, Bombay, etc to show respect to other cultures, it's a bit silly that we don't change Mecca to Makkah. On the other hand, while Mecca is the wrong spelling, it is the right pronunciation, and if the spelling changed to Makkah, many, many people would pronounce the word Mawk-kaw or Mahk-kuh, which is incorrect.
If I may put in my own 2 cents. I think the pettiness of this world is what is killing it. Who's life does it hurt that certain groups of people spell "Mecca," like that and other groups spell "Makkah," like that? Does it really matter as long as everyone understand the point? People just make things so hard.
I definitely think that most Muslims are offended that the word 'Makkah' is spelled 'Mecca'. But then again, it shouldn't a be a concern for people who aren't Muslims. I think that such matters are only thing that should be a concern for people who follow the Mulim faith and not for those who do not believe it.
Speaking as a Muslim, I have never been offended by the spelling of Mecca, Makkah, or whatever else you want to call it. The extremists who have an issue with that are our version of the Westboro Church. No one takes them seriously, or cares what they have to say.
I don't think the majority of Muslims are offended by the spelling of the word as long as it is not in a way that is offensive. People understand that many people cannot write in the Arabic alphabet and don't have a problem with them using their own alphabet to write the word.
Most Muslims are not offended by the use of the spelling 'Mecca' instead of 'Makkah'. Most Muslims are used to slang and know that the English vocabulary is not perfect. I think to most of them that it does not matter to them or in everyday life because they know what you mean.