Encrypting using FileVault on an Apple original SSD - any likely problems?
I recently did some research on this topic because I want to install an SSD in my Macbook as well.
In my case it would not be a factory fitted one but I doubt that there are big technical differences between original Apple SSDs and custom ones, apart from TRIM being available by default vs. enabling TRIM on my own with a custom SSD.

The most satisfying answer I could find was this discussion in the Apple Support Community.
The interesting posting basically says that a user tested random sectors on a FileVault2 encrypted SSD and found some that were indeed empty (all zero). In a similar scenario on a classic magnetic HDD no empty sectors were found.
The user concludes that this means that unused sectors remain empty on SSDs and that TRIM will correctly work with FileVault2.

The conclusion seems reasonable to my understanding and after installing my SSD I personally will use FileVault2 in good conscience.
However, this is not an official answer from Apple and therefore should be treated with care.