I've had zero issues with getting all but drm-ed LRF and drm-ed epub onto the iPhone.
For content from fictionwise/ereader, just buy in the ereader format; they sync down to the iTouch/iPhone rather quickly.
If you have just about any non-DRMed format, you can open it on your computer using Stanza, open up stanza on the iPhone, and the computer version of stanza should be listed in the shared books section of the iPhone stanza. Transferring takes about 10-30 secs. Easy peasy.
If you have DRM-ed mobipocked, you just need to go though an additional step of removing the DRM; once you do that, it loads up in stanza just fine.