What am I allowed to say?

Following on from Paula's comments on Whoppy's Whinge, I'm beginning to get really confused about what is "acceptable" to say, and what is deemed "politically incorrect" or "racist". I'm also a bit unsure of how Freedom of Speech sits with Political Correctness - explanations welcome!

While growing up, I don't remember taking skin colour/ religious beliefs/ disability/ ability/ sexual orientation/ shape/ size/ stereotypes/ appearance etc into consideration when making friends, or forming opinions of people, but I certainly told the odd Englishman/ Irishman/ Scotsman joke, or used labels such as Geordie/ Scouser/ Cockney etc, without a second thought. These days, it's deemed unacceptable to tell this type of joke, in case someone is offended.

Incidentally, can anyone tell me when a "joke" stopped being a "joke", and became politically incorrect? It definitely wasn't in the early 90's, when Essex Girl/ Blonde jokes were in circulation - and if these had come into circulation in this day and age, would they be classed as racist, or deemed acceptable as Essex girls are too thick to understand them? [relax, I'm an Essex girl, so it's okay for me to say that, er, I think - just in case, apologies to any Essex girl I just offended!]

Sorry, slight digression.... so, can anyone help me with my confusion? What am I, or not, allowed to say - for example, what is the correct description of someone who isn't very tall? Should I refer to them as 'vertically challenged'? Can I say someone is 'handicapped' or 'disabled', or is there a more acceptable term? Is the word 'fat' okay to use, or should I refer to my 'obese arse' rather than my 'fat lardy arse'?

Lordy, forget Wilkepedia, there should be an online politically correct terminology reference book for thickos like me!!!

p.s. can I say thickos?