I found that my backup is around 650MB or so, it seems like it backs up all the contact data, apps, sms, pictures, and settings but not Ipod media.
One thing I did notice though, maybe others may have known or not known, is that when you restore your phone you can choose to select the backup you want. I have seen people say that when they hit the x it corrupts the backup, just choose the last full backup (usually the one with the date) and you will be ok. It doesn't seem to put a date on ones that aren't complete. At least this seems to hold true on PCs not sure about Macs.
I have an acer laptop. it was recently hit by a virus. iTunes always said 'backing up iTunes' when i connected my iPhone. Will i be able to retrieve my old library?