Can you read a USB flash drive from an Android phone? I have a Samsung Galaxy S (i9000) with Android 2.1.

I am wondering if it can read a USB flash drive. It will be really useful to copy files across USB drives so you don't have to carry laptops around.

I think if there is a cable/converter to plugin a USB into the phone and there is an app to read the connected USB drive, I don't see why it can't be done.

The problem is do the cable and app exist?
Yes and Kinda.

You're talking about USB On the Go (OTG) and Android is capable of it but it's not officially supported.  I couldn't find any apps in the app market that handle it but here are a couple people that got it working:

http://sven.killig.de/android/N1/2.2/usb_host/ (this guy basically turned his Nexus One into a whole work station)

http://www.tombom.co.uk/blog/?p=124

That second tutorial tells you to hack a cable but you can actually buy an OTG cable so this isn't necessary.

Also, I've seen some phones claim they are going to come out with USB OTG a while ago but I don't recall anything ever materializing.