...so what is "Account Planning" within Advertising?

Well, if you're interested, here is the official definition, updated in 2001...
http://www.apg.org.uk/about-us/what-is-planning.cfm

Account Planning Department:
The Account Planning department used to be called the Research Department. This is the department that came up with the famous line of Americans having “2.3 children”. So the British came up with the idea of Planning. Planners are responsible for the consumer’s view point. They are the eyes and ears of the consumer. They advocate this view point to the client, the creatives, and the media department. They are also a very creative bunch. How do you get into the heads of athletic high school girls? Hang out with some on a HS basketball team, become a member of the team, go everywhere they go, and do everything they do - for a solid month.

Job description of Account Planners:
Account planners play a key part in developing advertising campaigns for a huge range of products and services. Their role is to combine the attitudes and interests of consumers, clients and creatives to help produce effective advertising. Acting as the voice of the consumer within an agency, planners use research data to identify ideal audiences and optimum methods of communication. They combine market data, qualitative research and product knowledge within a core proposition to enable the creative team to produce advertising ideas that resolve defined business problems. With increasing public awareness of marketing strategies, a key challenge is to develop innovative ways to reach consumers.

Typical work activities may include:

• liaising with clients to identify specific business problems and develop ideas;
• communicating with colleagues within the agency, such as creatives and account managers, in the process of developing a campaign;
• analysing a wide range of information in great detail to gain a comprehensive context to advertising strategies. This may include demographics, socio-economics and the market for the client's product and market share;
• commissioning research from outside organisations, as required, to inform advertising strategies, using both qualitative methods, such as focus group discussions and structured interviews, and quantitative methods, such as demographic profiling and questionnaires;
• running qualitative research groups;
• monitoring cultural and social trends, and their impact on consumers' attitudes, behaviour and perceptions, using a variety of market and research data;
• finding an 'angle' on a specific product or service on which to base an advertising campaign;
• researching the product or service, which may involve gaining technical or specific knowledge;
• reconciling the differences between consumers' current perceptions of the brand and the way in which the client wishes the brand to be perceived;
• meeting the client to learn the background to the brand and advising on possible approaches or adaptation of approach to the target market;
• providing the creative team with a clearly defined brief that contains concise information on the product, audience and strategy enabling them to develop creative ideas that may be applied to a variety of media channels that will promote the idea most effectively;
• presenting conclusions and ideas to clients and other agency staff;
• analysing and interpreting customer response and sales data in order to evaluate the effectiveness of the campaign.

http://www.ciadvertising.org/SA/spring_03/382j/MDEMORI/stanley%20pollitt/AP.htm