Will upgrade of iOS delete any of my videos or photos? I wanna upgrade to the latest iOS 5 and wonder if my existing photos and videos at Camera Roll will be still on device. Is that right? Or I have to backup them before upgrading? If so, how would I do that - iTunes doesn't seem to backup those things at sync.
The photos and videos in Camera Roll are not synced with iTunes - they are, instead, the "raw" photos meant to be synced to the photo-organization tool you use - whether that's a set of folders in your filesystem, or iPhoto.

In other words, when your iPhone is mounted as a "USB camera" (on Windows) or when iPhoto opens (when plugging it into your Mac), the contents of the Camera Roll are what you see.

That being said, iOS 5 will delete all of this data during the upgrade, so you absolutely should back them up before going forward! Assuming you're using a Mac, plug in the phone, open iPhoto, and import everything before doing the upgrade.

(And I recommend making a backup of your phone, transferring purchases, and then syncing everything you can with iTunes before going forward... the iOS 5 upgrade appears to be a very destructive procedure.)