Believable:

I looked for the same thing for a while.  I finally settled on Comic Life from Plasq.com.  It does exactly what you described, and I think quite well.  You can drag and drop your images from a directory into frames, then add text bubbles, etc.  There is a lot of functionality to change the fonts, colors, sizes.  The text auto centers in the bubbles, which is the key factor for work flow.

It's got a couple of rough edges, but the pick of the litter from what I've seen.

There are both PC and Mac versions.  Twenty five bucks for the standard and thirty bucks for the advanced version (more fonts, templates and pre-designed styles).

Klebnor

Lotus 123 ~ S-Render ~ OS/2 WARP ~ IBM 8088 / 4.77 Mhz ~ Hercules Ultima graphics, Hitachi 10 MB HDD, 64K RAM, 12 in diagonal CRT Monitor (16 colors / 60 Hz refresh rate), 240 Watt PS, Dual 1.44 MB Floppies, 2 button mouse input device.  Beige horizontal case.  I don't display my unit.