My caveat to the liberals would be to warn against cargo-cult methodology; that is, they may try to ape all the "obvious" keys to the success of Rush without understanding what actually made the show a success. Whatever Limbaugh's true level of political committment, he was an organic phenomenon. If there were a market for liberal talk, I think it would have been filled already. TNR does, unfortunately, quicly dismiss some of the serious theories explaining the relative successes of Limbaugh and, say, Jim Hightower.
One is that conservative and liberals have different media habits and needs. Demographers could tackle this question better than I.
Another, favored by conservatives themselves, is that "the networks" already fulfill the liberal media needs. This isn't so much a matter of media bias as it is of media content. To focus on radio, I abhor the slant of NPR, but it is objectively more interesting than Limbaugh's show -- more variety, more solid content. I see why educated, wealthy liberals listen to public broadcasting: high quality, great human interest stroies, effective use of the medium. I honestly don't think most liberals are going to switch off Diane Rehm to listen to Janeane Garafolo; the former is informed, intelligent, and consistent (though with perhaps the worst radio voice ever), while the latter is ignorant, embarrassing, and poorly equipped to be entertaining every day. (One of Limbaugh's great strengths is that he can deliver relatively sophisticated political analysis without sacrificing entertainment value.)
So liberal talk, like the metric system, is always this close to making mainstream success. Maybe. I'll wait until they roll out some content. They seem to be putting a lot of faith in the ability of unproven, loose-cannon types like Al Franken to attract backers/sponsors. But radio takes a lot of discipline, and talk radio all the more so. I should know: I tried briefly to run the morning-talk format in my days at a certain public broadcasting station in the backwaters of Northern New England. I had ideas. I had determination. Unfortunately, I just plain sucked.