[magnus]

Well, rather than heeding the advice of all the armchair lawyers -- here's Google's TOS regarding Ad Placement:

Originally Posted by Google TOS

Ad Placement

• Up to three ad units may be displayed on each Web site page.
• A maximum of two Google AdSense for search boxes may be placed on a page.
• A single link unit may be placed on each Web site page, in addition to the ad units and search boxes specified above. Link units are considered to be 'Google ads' for purposes of these program policies.
• A single referral button per product may be placed on a page up to a maximum of 4 buttons, in addition to the ad units, search boxes, and link units specified above. Referral buttons are considered to be ‘Google ads’ for purposes of these program policies.
• No Google ad may be placed on any non-content-based pages.
• No Google ad or Google search box may be displayed on any domain parking websites, pop-ups, pop-unders, or in any email.
• No Google ad may be placed on pages published specifically for the purpose of showing ads, whether or not the page content is relevant.
• Elements on a page must not obscure any portion of the ads, and the ad colors must be such that any ad elements, including text and URL, are visible.
• Clicks on Google ads must not result in a new browser window being launched.