Hi.
When we plug in USB devices on our computers, the computers automatically searches and installs the appropriate drivers for the devices to enable you to use them. For the arduino board, if we plug in devices (eg. Bluetooth dongles, webcams) via the USB host shield, how are we going to upload the drivers into the arduino board? Are we suppose to write the drivers ourselves?
Thanks.
Pretty much. Unless someone has already done it.
If the drivers for a specific device is already present, how do I upload into the arduino to ensure that the device plugged in will work?
See the last line of my signature.
Um, that doesn't really help.
An arduino is just a microcontroller it can't understand drivers written for windows, mac or even Linux. Unless some kind soul has done all of the leg work and shared with the community, you'll need to write the releveant code to allow your arduino to communicate with the USB device. Remember that your arduino doesn't have an operating system like a PC. It just runs one lump of code over and over and over.
Out of interest - what USB device are you looking to connect?
I'm thinking of attaching a USB dongle and a webcam to the arduino. Correct me if im wrong, but does drivers = libraries in this case?
And also you mentioned in your sig that its not feasible to connect a camera to the arduino and send pictures wirelessly. Is the reason due to storage space of the Atmega chip? Do you have any recommendations for a microprocessor which is capable of controlling servos and sending pictures wirelessly?
Tiny RAM, limited processing no multitasking. We're talking about something that is happy controlling the fron display on your microwave. Don't get me wrong, I love Arduino - it's just that it's got its strengths and its got its weaknesses.
Have you considered something like a beagle board or MimoPlug? OR even a wireless IP webcam?
Hey I'm going to do exactly that! (your signature) Using the PTP library with tiny thumbnails at least.
Wait... Did you just call me a fool? 
Why do you need these 3?
Tiny RAM, limited processing, no multitasking
Just read the data 1k at a time from the PTP library, write it to XBee.
Yes, given enough time and patience it is possible to hammer in nails with a screwdriver. I suspect the OP wants to deal with video.