Ok, The idea of changing TAB key wasn't a good one. Although the fact remains that CTRL+I functionalty could be moved to another key, And the current CTRL+R could be switched to CTRL+I and changed to italics. This would not break any old script, And perhaps it could be configurable in the options.
At first not many people would use italics, Probably because mIRC has deprived them of it for so long they are used to emphasising words differently. Either way i'm sure it would be a lot more used than reverse text is.
