Advertising inventory is under attack from two sides:
Internet: links drives up prices
TV: time-shifted watching (through VDRs) reduces the 30s-commercials market

Inventory on the other hand is growing:
Video games:

• in-game advertising, as a form of product placement that has already emerged form infancy stages. Both Massive and inGamePartners provide services. Nielsen Entertainment trials a metering service in cooperation with Activision (for inventory) and Chrysler (for displaying the Jeep brand).

TV:

• Sponsoring of reality TV shows.
• The rise of the 10s-commercial.

Internet:

• Extending the reach through Google SMS (search from mobile handsets).
• Google Desktop (increase inventory to include PC hard drives) and Enterprise Search (to include intranets).
• Personalisation (such as MyYahoo!) en localisation (Yahoo! Local, Google Local) to drive the value.
• Also: an enhanced search experience. Specifically: MSN will debut a new search engine within the next few months, Vivisimo introduced Clusty and Idealab launched Snap.com. Google, Yahoo! and Ask Jeeves continuously improve their search engines.
• Extending search databases by including books & magazines (Google Print, Amazon Search Inside the Book) and science (HighBeam).
• Advertsing in e-mail (e.g. Google's Gmail) and along web logs (Google's Blogger service).

There does not seem to be a shortage of inventory, but to tap all the above requires capex and willingness to take risks.