Yeah, my wife uses "sweetie" online too. If you Google "sweety" you'll get 11.5 million hits. "Sweetie" nets 13.4 million, so it looks like it's a slight favorite even though both forms are in common use.
Personally, I like saving a letter and using "sweety." But I think it might be a "form of endearment" thing ... the "ie" ending just seems to be more informal and personal. Kinda like how she spells her name as "Sandi" rather than "Sandy," but then, I do believe there may be a gender connotation in the various spellings of that name. As with "Kris" and "Chris."
In German, a distinction is made between "Sie" and "Du." Both are second-person pronouns, but "Du" is considered more personal and would be used to refer to family and close friends.
So you're probably right. No special rule. It's just an English language peculiarity.
Please help me.
I chat with my girlfriend online and call her 'sweety' all the time... and she types 'sweetie'... then one day I think, 'wait a moment. im wrong' so I do quick search on the google and find that, 'sweetie' is correct... and I'm wrong. Problem solved, I type sweetie now... yey! But then I get this nagging feeling about why it isn't sweety.
If sweat becomes sweaty, where does sweet get off in using 'ie' instead of 'y'. I looked around the net trying to find why its like this, and see answers such as "it's just right" and "it looks right". The closest I got was 'sweety' and an alternative spelling to sweetie.
So, is that the answer... sweetie is a word... sweety isn't. No special rule, it's just one of those things as an english speaker you have to remember.