If I copy text from Safari or another app, is it generally copied in rich text format, or plain text format? And if rich text, is there any way to paste it formatted, or see the HTML source of the copied text?
Text is copied in rich form in iOS 3+.
You can try it by copying some text in Safari and pasting it to Mail.
But pasted text format will depend of the app. For example, the SimpleNote app only paste in plain text format, since the service doesn't not support image and text styling.
There is no native way to see the copied text in HTML form. You'll have to find an app to do that.