Alright, so level with me...

[Remathilis]

It's also the trade dress, such as the formatting of the monster statblocks, format and layout of power statblocks, use of icons, and even use of the term "Dungeons & Dragons".
It's going to be hard to advertise a book as compatible with 4e without using the term D&D.

Pathfinder does.

"Compatible with the 3.5 revision of the World's Best RPG."

 

[S'mon]

It's also the trade dress, such as the formatting of the monster statblocks, format and layout of power statblocks, use of icons, and even use of the term "Dungeons & Dragons".
It's going to be hard to advertise a book as compatible with 4e without using the term D&D.

Stuff that you can't see before you buy the product is not trade dress under UK law - "monster statblocks, format and layout of power statblocks, use of icons" would be covered by copyright only, not by Passing Off/trade dress.
People advertise compatibility while staying OGL compliant all the time - eg "compatible with the 4th edition of the world's most popular roleplaying game". Of course only those already 'in the know' will know what you mean, but you're not trying to sell to the general public.

 

[pemerton]

It's going to be hard to advertise a book as compatible with 4e without using the term D&D.

Well PF, and all the retro-clones, seem to manage this. There are a range of euphamisms, starting with "the world's favourite fantasy RPG".

EDIT: somewhat ninja-ed once I open the next page.