In the guild gump there is a field in which to enter the url of your guild website or guild forums. This then appears in your guild listing on UO.com. Many, if not most, guild websites and guild forums have banner ads."With the exception of the sale of in-game items for in-game items or services, you may not market, promote or advertise anything, or make any other form of solicitation (including pyramid schemes and chain letters) through the Ultima Online service or Ultima Online web site."
I give it a week until we see reports of people getting suspended and banned because griefers are searching the UO.com guild listings for guilds with links to sites with banner ads and paging on the guildmasters.