In regards to iTunes content, an iPhone can be synced with an iTunes library on a single computer only, and the same applies to photos transferred to your iPhone - from a single computer only. When syncing iTunes content and photos from another computer, the existing iTunes content and photos transferred from another computer on your iPhone will be erased first.
You can sync contact info, calendar events, and bookmarks with the supported applications on multiple computers.
I know that I cannot sync my photos or library and that is just fine with me. What I really want to do is sync my calendar and contacts from both my macbook and my work pc onto my iPhone. How do i do that?
Disable automatic syncing for your iPhone when connected to iTunes, which is selected under the Summary tab for your iPhone sync preferences.
If you are currently syncing contact info and calendar events with the Address Book and iCal on your Mac, you are set there.
On a Windows PC, the iPhone supports syncing contact info and calendar events with Outlook 2003 or 2007. This is selected under the Info for your iPhone sync preferences with your Windows PC.
If you are currently using Outlook for contact info and calendar events, you will be provided a merge prompt when syncing this data, which you want to select.
Afterwards when syncing your iPhone with the Address Book and iCal on your Mac, you may be provided a merge prompt again and if so, you want to select it. After doing so, your iPhone will be synced with the Address Book and iCal on your Mac and with Outlook on your PC via the iTunes sync process.
It's not recommended to sync data (contacts and calendars, and bookmarks) between multiple computers. It can result in data loss or duplication; I know this from personal experience.
I would recommend moving your important data to one computer, organized between separate calendars and groups, or using Exchange ActiveSync or MobileMe.