It is necessary to understand what an MP is. Technically, MPs are legislators, who vote in parliament on bills and motions. However, that is now a ceremonial role, with no effect on the government or the country. The position of MP is an apprenticeship to the ministers or shadow ministers. In the same way that apprentice footballers have the irrelevant job of cleaning boots to keep them in their place and instill obedience, MPs have the irrelevant job of turning up for votes in accordance with the whips' orders. (They also have the even more irrelevant job of acting as a kind of Citizens' Advice Bureau for their constituents, but I'm not sure whether that's to give them practice running an office, or for some other nefarious end.)
Therefore being an MP is, in itself, no more desirable a prize than being a football club boot-cleaner. OK, it is rather better paid, but that's not much of a pull for the average TV star. It doesn't even provide much publicity - does anyone remember hearing much about, say, Gyles Brandreth during his time in the House?
The position of MP is only meaningful as a step towards the front bench, just as a football apprentice is only in it for his chance at the first team.
The advantage that a celebrity has is recognition. But while recognition is an advantage while rising in a political career, it is a handicap as the top approaches. When it comes to a party leadership contest, the most important factor is actual power - Gordon Brown was able to succeed Blair because he was already powerful. But after that the biggest advantage is not being disliked. In a leader-of-the-opposition contest, the ideal is for the general public not to know anything about you at all. That worked for Cameron, Duncan-Smith, Hague, and Blair. (Howard was an exception, but he was never intended or expected to win a general election).
Therefore, if a soap star or newsreader wanted to succeed in politics, they could probably get selected as a candidate, probably get into parliament, would probably be able to rise quite rapidly to a junior government role - PPS, or Minister of State, but would find it quite difficult to reach a major cabinet position, because of all the people who didn't like them.
We see glimmers of the future in Brown's elevation of Alan Sugar. Not being an MP, Sugar is out of the main political career path, but if his entrance had been a bit more planned he could have got a seat in the Commons and been better situated for a less temporary role. He would have gained his current position in his first parliament, but the odds would not be in his favour for further promotions.
The celebrities with the greatest advantage would be those whose public roles gave them credibility on political issues; the interviewers, newsreaders, and pundits. Robert Peston, say. Or more lightweight figures like Nicky Campbell, or Rantzen twenty years ago rather than now.
I suspect the step has been slowed by the reluctance of party grandees to admit potential rivals with such inbuilt advantages. In today's environment, however, the potential candidate only needs to announce his intention, and the onus is on the party to explain why they are refusing him.
The key question, as I said, is whether celebrities would want to abandon their media careers for politics. Most wouldn't, at the moment. We have seen that those that did, like Martin Bell and Robert Kilroy-Silk, set their expectations too high.
But once a few more oddities have blazed the path, the game changes, because ambitious young things with eyes on the greasy pole will see media as the career path to the cabinet - not in one explosive burst but by working through the ranks just a bit quicker than the normal rate. Rather than hanging around the think tanks and party research offices, they'll be driving with all their ambition into the local TV studios, working the system with whatever influence they have at their disposal to get them the foothold of popular visibility, so that they can then switch to party politics with a head start over their anonymous rivals. Rising stars will be guided by their political mentors through tame TV or newspaper departments. The end result is that the two sectors just merge into one. Media figures will expand their reach from the political areas they currently own (such as the London Mayoralty) to those which are currently held against them by the party machines. Shifts from media to politics and back, like Kilroy-Silk's will become commonplace.
What's most important is not the effect on politics, but the effect on media. That is always the way - politics stays the same, but what it touches gets polluted. The Robert Pestons and Jeremy Paxmans (Paxmen?) of the future will not be doing their jobs because that is what they set out to do - they will be doing their jobs to get the public reputation that will put them in high government office. The detrimental effect on the media will be equivalent to the detrimental effect on Parliament of making an MP job nothing more than a stepping-stone. The lines have been blurring for some time.