The Galaxy S I9000 messaging app automatically converts long SMS messages to MMS. How can I disable this?
You can't do so with your samsung official messaging application, all of what you can get is a pop notification that you are now in the MMS zone (Converting to multimedia message...).
I would recommend Handcent SMS application, available at the market, and has an option for not converting texts (SMS) into MMS
I have found this feature request with a long discussion in Android issue tracker.
From what I understand this issue was fixed long ago in stock Android starting from Froyo (2.2). But manufacturers don't tend to include this fix in their devices because it will prevent them to make money. Here is one of the relevant comments from the issue:
http://dailymobile.se/2012/02/10/the-top-5-android-anti-features/ interesting article that features this problem.
Listen, this ISN'T a bug in Android. Google fixed this years ago, manufacturers/networks don't want to include it, don't you get it? As the article I included above says, who benefits converting your FREE SMS to a PAID MMS? Not Google, the networks and manufacturers. They don't want to include it, it makes them money not too. So you're left with two options if you want to avoid it, root your phone or install an app like BigSMS.
The author of the comment recommends BigSMS as an alternative to preinstalled SMS application, but I installed Android Messages by Google and it solved the issue: my long SMS was split on 4 parts and sent without converting to MMS. See also this relevant question: "Converting to multimedia message" when forwarding large SMS.