Bionic seems to be aimed at the high end business market with some fun (media, qHD, LTE, Tegra2, dual core, games, possible webtop - though the 512mb RAM don't add up to webtop)
'daytona' (X2) for those that like to keep it light and thin and those that don't want to pay for 4G (no LTE). Price will possibly be lower than the other 3. Seems like a refresh of the X, just like how the D2/R2D2/D2G was a D1 refresh.
D3 for regular business types/enterprise and ppl that need a physical qwerty and not media focused (no LTE, 4"?), though I am sure the Pro will most likely get a refresh as well later in the year.
'targa', if real, will most likely be DROID branded as well, with a new moniker. That one will be aimed at new adopters that wants the monster phone of the moment. (LTE, 4.3"?, The same group that bought the X last year, and the D1 the year before. I'd say that would be the last one to come out, right before the DROID Pro refresh.)
And if it is coming out at the end of the year, I really wonder whether it'd have a similar quad core processor that the NGP (which would most likely launch in November as well) is slated to have. From TI of course. OMAP5?