Isn't the cell phone screen kind of too small, anyway?

It is quite small, indeed, but not too small to read from. And tx2ph.com lets you fine-tune the font size, and even rotate the text 90°, so the lines run along the longer side of the screen:

When you create your free account, you'll be able to chose the display mode that is right for you: either an "image" mode, in which the book text is "printed" into an image, allowing for arbitrary page layout, or "plain text" mode, in which your cell phone's default font and page layout will be used. PDAs, Blackberries and mobile phones with big displays may be better off using the default font for reading books, so the plain text mode is now the default one for these devices.

While reading an ebook, you can always change the output mode by selecting the Mode link at the bottom of the page. The output will cycle through all available modes.