RSS to mobile
 Windows Mobile
The PRSS reader is the one of the best readers for these devices. It is a free download from http://code.google.com/p/prssr/. It lets you view RSS feeds directly on Windows Mobile devices. It can read several messages in a row without having to return to the message list. It also supports images and other html extensions. It can automatically update the RSS feeds when you sync the unit and displays the message status on the today screen.
Here is a list of the top ten RSS readers for Windows Mobile devices.
If you are using Windows on a portable device then Internet Explore 7.0 from Microsoft has an addin that can be used to directly read RSS feeds. It works very well and one of the few that supports categories. Firefox Thunderbird and SeaMonkey can also read RSS feeds directly.
 iPhone and iPad
NMD Parser - This site is in Russian but since it is a Google site there is a Google translation button you can hit. This is a free program that will translate an RSS feed to FB2 format. There are several eBook Readers that can read FB2 files. Generally they are either FBReader or Cool Reader based. These two readers are also available on more general portable devices.
 RSS to eBook
News to eBook will convert an RSS feed to ePUB or MOBI formats for use with an eBook Reader. This is a web application and should work for any device. Also works with Atom feeds and Google Reader subscriptions.