Properties and methods are given "positive" names as a matter of convention, and to make reasoning about Nil easier.
In Objective-C, the question "should I name my method isEnabled or isDisabled?" has a reasonable answer - you consider what makes most sense (or any sense) when called on Nil.
This is probably a good rule for other languages too - "negative" booleans can lead to double-negatives and unreadable code.
Even with a positive naming convention, you will have conditionals for the negative case, e.g. if (![str isEqualToString:@"myvalue"]) and falling into its block is not desirable when str is nil. Personally I have had plenty of bugs around things being unexpectedly nil, but I can't think of a case like this where I executed a block unexpectedly.
In Objective-C, the question "should I name my method isEnabled or isDisabled?" has a reasonable answer - you consider what makes most sense (or any sense) when called on Nil.
This is probably a good rule for other languages too - "negative" booleans can lead to double-negatives and unreadable code.