Nick Wingfield wrote an interesting article on the Wall Street Journal site called Time to Leave the Laptop Behind and I think he did an excellent job at articulating the options, trade-offs, and current state of mobile devices for the road warrior. Mobile phones today can be extremely powerful and capable so that some people are using them to work on the road. I personally have traveled a few times with only my mobile phone and a REDFLY Mobile Companion and think it can be done. However, I am not sure that the minor savings in weight is worth the compromises you need to make a mobile phone work for you.
Some of the compromises are in regards to the display and keyboard sizes, but something like the REDFLY (check out some of my thoughts) can address this area. The REDFLY only works with Windows Mobile devices though, so that limits your phone selection. I also find that the web browser can be limiting on a mobile device and there are more steps involved to do things like basic photo editing for post creation. It is also often impossible to connect to a hotel's wireless network with a phone due to the sign in requirements, but if you have a good 3G connection this can also be addressed.
If you have a Nokia S60 device, I actually found that it can be a better road warrior solution than even the REDFLY if you connect to your hotel TV and use a Bluetooth keyboard. I have not used the iPhone with a TV out cable so I don't know what it projects on the TV/display, but the iPhone still needs better Bluetooth support to be a real road warrior tool for extensive text entry.
I think a combination of the laptop and powerful mobile phone as the article mentions is probably the best solution. One this current business trip I find that leaving my MSI Wind netbook in the room for more extensive writing while taking my G1 or Treo Pro to the conference and meetings is a solution with very little being compromised.
Can you travel with just your mobile phone and still get everything done you need to?