Lindberg, Revell, AMT and Johan have been in a slump for many years, however Lindberg and Revell each came under new ownership in early 2007 and have really stepped up production.
Lindberg has re-issued the old Auburn, Cord and Lincoln kits however those kits are notoriously bad and take a lot of work to build decently. They also have a '40 Ford coupe, '34 Ford pickup, and several '53 Fords as well as some 60's era cars, all of which are supposed to be pretty decent kits.
Revell has an all new tool '49 Merc custom & '58 Impala, as well as a number of '30-50's Fords and Chevys. They also re-issued their 41 Chevy pickup and '50 Ford pickup with some new parts last year. Revell has the Monogram line so hopefully they will start to re-issue some of the high end cars from the 30's like the Duesenberg.
AMT practically came to a screeching halt in 2007 but has new management this year, you won't see much until late in the year but they are planning on bringing back a '49 Ford, '51 Chevy convertable, 'and 60 Ford Starliner as a start. 2009 should be when they really get up to speed and they say they intend to re-issue some of the stuff that hasn't been available in many years. They had a nice 1930's Lincoln and a 4 in 1 model T kit within the last few years so hopefully we will see those again.
Johan is basically on life support right now, it came under new ownership a few years ago but apparently was in much worse shape than the owner expected and releases have been pretty slim.
It's been a little rough for model cars the past several years but it looks like its getting a lot better.