Hijacking trade marks for use behind the scenes in search engines or embedding into website coding may amount to trade mark infringement (at least in Australia).
In Accor Australia & New Zealand Hospitality Pty Ltd v Liv Pty Ltd  FCAFC 56, the respondent was sued for trade mark infringement. One contention was that the respondent used the appellants mark, HARBOUR LIGHTS, in the source data (including as a meta-tag) of its website.
The Court observed that although the meta-tag itself is not displayed on the screen, it is used by a search engine to determine which search results would be displayed. That use was, "effectively, use as a business name for a business" and operates "as a badge of origin to distinguish Liv's services from others" and would amount to "use of a mark substantially identical with and deceptively similar to each of the registered trade marks in suit".
So, in Australia, hidden advertising practices may constitute trade mark infringement. This differs to the position in New Zealand (set out in Intercity Group (NZ) Ltd v Nakedbus NZ Ltd  3 NZLR 177). There, Nakedbus used the keyword "inter city" and variations of it via Google Adwords to direct consumers to its website. Asher J accepted that "if the "use" could not be seen by the consumer it could not be "taken as" anything, let alone "taken as being used as a trade mark". Therefore, any such use would not constitute trade mark infringement as the consumer would have had little, or no, knowledge of how the advertisement came to appear.
Whether that position will remain is yet to be seen.
Because branded keywords and meta tags are invisible to the end consumer (assuming the average consumer does not trawl through a website’s source code), the position to date is that that branded keywords and meta tags are not taken as trade marks, despite the fact that this is precisely how they are being used by the advertisers themselves.