This is an interesting thing we caught recently. When sharing photos on iOS4, you normally get the option to send via MMS or email.
One thing you may encounter though is that when you try to share more than five photos, you only option is to send via MMS. This is shown in the photos below:
However, if you are sending five images or less, you will see that the you are given both the option to share via MMS or email, as shown by the following photos:
Not sure on why the restriction is there, but consider this nothing more than an FYI or a little iOS4 tip for when you are sending photos to friends and family.
Note: How To Send Photos In Full Resolution From Your iPhone – Same rule is in iOS4 as 3.0