Both MobiPocket and
eReader have support for lots of types of phones. I only linked to eReader because MobiPocket is almost dead. The existing software works (well in many cases) but most likely won't ever be upgraded.
There is a trend for newer Readers to only work with smart phones (Kindle replaces Mobipocket, and B&N Reader replaces eReader). The order of introduction seems to be iPhone, BlackBerry, Android (with Android not out yet). As has been mentioned, HTC may be introducing its own ePub-based service for its WinMobile phones.
In my view, Android is the wave of the future. There are good DRM-free Reading apps already, plus a beta from eReader, and soon everyone that matters will have an Android Reader app (see
Reading on Android).
All the above probably require you to convert from RTF to something else. The only exception I am aware of is
uBook for WinMobile.