With Cocoa, you need to read the file into memory, append the new dictionary object, and write it back to the filesystem. If you use an XML plist, you could pretty easily parse it and incrementally write to the file, but it'd also be quite a bit bigger, so it's unlikely to be worth it.
If rewriting the plist is taking too long, you should investigate using a database instead (perhaps via Core Data). Unless the file is huge, I doubt this will be an issue even with the iPhone's memory capacity and flash write speed.
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