Can I select a specific block of text in a UITextField?

With UITextField, you cannot. But if you see the headers, you have _selectedRange and others that might be used if you add some categories to it 😉

Update for iOS5 and above :

Now UITextField and UITextView conform to UITextInput protocol so it is possible 🙂

Selecting the last 5 characters before the caret would be like this:

// Get current selected range , this example assumes is an insertion point or empty selection
UITextRange *selectedRange = [textField selectedTextRange];

// Calculate the new position, - for left and + for right
UITextPosition *newPosition = [textField positionFromPosition:selectedRange.start offset:-5];

// Construct a new range using the object that adopts the UITextInput, our textfield
UITextRange *newRange = [textField textRangeFromPosition:newPosition toPosition:selectedRange.start];

// Set new range
[textField setSelectedTextRange:newRange];