IPhone syncs only one way

What are you trying to sync back?

It depends on what you are synching. Music, videos, photos sync one way. Calendar & contacts are bidirectional. Notes don't sync at all. If your calendar and contacts are not syncing both ways then something is wrong; probably bad data in the calendar or address book.

"Music, videos, photos sync one way..."

This can't be strictly true can it? My iTunes should be informed when I've listened to a Podcast on my iPhone!

I have noticed that iTunes doesn't register a 'play count' or 'last played' date for tracks I've listened to on my iPhone which is really irritating - is this just my phone or a common problem?

I'm also wondering this...reason being, I have added some new contacts to my iPhone which I want to add to my outlook on my computer for when my replacement phone comes in.

On a Mac there are a couple of ways. One is to move the address book data file to the desktop (for safety) and create a new one with at least one entry, then sync with the iPhone. This should replace the desktop with all entries on the iPhone. Another way is to create a new group in address book, set it to sync (in iTunes), then copy a few addresses at a time into it and sync. When the sync stops working you have narrowed down the search to the bad record(s). (For techies, a binary search algorithm is faster).

On a PC you export the Outlook PST file, the import it. Alternatively, look at the first few entries in the contact list. It is usually one of those. To check open a card and save it; if it is bad the save will fail.

One thing to try is to create a new user account on your computer.
Log out of your main account and into the new account.
Open address book and create a test contact.
Open ical and create a test event.
Now open itunes and go through the initial setup.
Connect the iphone and go to the info tab.
Set the iphone to only sync contacts and calendars.
You should get a message about the iphone being synced with another computer.
Choose merge data.

If this works, there might be an issue with the data on the computer.
If this does not work, you might want to restore the iphone and set up as new iphone.
Here is an article from the apple support website that might help out.
http://docs.info.apple.com/article.html?artnum=305744

Hope this helps.