is it more beneficial to place various areas of the application in seperate databases or in one database file. for instance putting forum tables in a single database file and putting the blogs tables in another db ??

Separate databases most likely don't seem sensible but, presuming SQL Server 2005 or later, I'd encourage you to definitely consider using schemas to realistically separate these functional areas. See Buck Woody's article SQL Server Best Practices: User-Defined Schemas like a beginning point.

is it more beneficial to place various areas of the application in seperate databases or in one database file

Ah - waht exactly would you request here? Is about DATABASES or about DATABASE FILES - a database might have many filegroups which each might have many files.

Is dependent on size and IO needs. I've come across databases with 28 categories of multiple files each, to optimize the IO bandwidth from the underlying SAN that was restricting every LUN (which had multiple files per LUN) to 256 outstanding demands.

when you get into greater finish needs, which makes sense. Likely for you personally it doesn't, as the working signifies you do not fully realize databases, which means you wont focus on a multi terabyte top end system )