Blogging for a little bit of cash might be more difficult as time goes by. The reason is that links are being changed by webpage hosts to show as what is called 'nofollow'.

See Krishna's blog entry Blog software makers on "nofollow"

This will mean that it will in the future get more impossible to 'hide' advertising post entries, as they will be the only ones, or mainly the only ones without the nofollow attribute in the code.
Going further, this again means that blogs that advertise will/might never get PR. The efficiency of the links from the advertisers point of view will make them vulnerable to being detected by Google.

To understand this a bit more you have to realise that the reason most advertisers pay for ads is not because they are expecting you to send them paying customers from your blog, but rather to increase the PR of their own website, making it come out higher in the searches. They do this by having lots of incoming links, in this case that they pay for.

Google sees this as 'cheating' as so will try to get it stopped. So, if you are making plans to retire and blog for a living, bear it in mind that paid blogging might not be as profitable in the near future, as the advertisers will not pay unless the links are worth it, in terms of 'getting seen' on the internet, and particularly in searches.

Google is fighting against buying links for quite some time now. Not only Google, but MSN, Yahoo, and all other search engines support them. All the major blog software makers too support them. Even Bravenet templates are with "nofollow". C-box, and other tags/chat boxes are "nofollow".
It does not mean the end of earning for professional bloggers. Yes, if they pay you for "LINK JUICE", well, some may shy away the moment you mention "nofollow". If they genuinely want ads/traffic, they will pay you.
Further, you need a good PR to be paid well. PR cannot be bought. You toil hard for that. What is the fun if that is gone, by earning a few dollars?
As to hiding text, Google is dead against it. Hiding text / URLs can be done in many ways, without being visible to your visitors, but visible to SE crawlers/robots. If Google detects that, You lose not only PR, but a Google ban and your pages never will appear in Google searches.
There is NOTHING for getting paranoid. If we spread awareness, and when everyone stops selling links, you will earn as usual. It is only a small trouble worth taking for a good future.