Recently during a customer POC we were required to call OBIEE Dashboards from a Portal (happened to be Lifray), passing prompts.
Historically you had to build up the URL yourself to include the relevant parameters in the URL - a painful and error-prone process. Venkat provided an excellent blog entry on URL parameters (have a look at the list he maintains on spreadsheets.google.com).
To simplify life, an interesting feature was introduced in 10.1.3.3 which makes the creation of the URL to provide the prompt parameters a lot easier.
To enable this functionality you need to add the following to your instance config.xml (and bounce the Presentation Server process):

To quote the documentation:
"This Prompted Link is intended to support both manual and programmatic manipulation of that link and also aims to provide a simple URL-based API for generating and customizing Dashboard content. Unlike the Bookmark Link, this link is dynamic, which maps to no saved catalog content.
When configured to show this feature, the Dashboard Page Options menu will show a "Create Prompted Link" option. Selecting this option will generate the Prompted Link and the user will see a message indicating, "A Prompted link capturing the prompts and values of this page has been created."
This is the Prompted Link syntax:
, Action=Navigate, Column1 (col1), Operator1 (op1), Values1(val1), Column2 (col2), Operator2 (op2), Values2 (val2) ...
Values are separated by pluses and to further simplify this syntax the operator parameter is not needed for equal (eq). Double quotes are also optional if there are no white spaces inside the values.
These are Prompted Link examples:
...Action=Navigate&col1=Products.Color&val1=Armory
...Action=Navigate&col1=Products.Color&val1=Armory+Clear
...Action=Navigate&col1=Products.Color&op1=bet&val1=Armory+Clear
...Action=Navigate&col1=Products.Color&val1="Armory"+"Clear"&col2=Periods."Month"&val2="04/01/1998%2012:00:00%20AM"
...Action=Navigate&col1=Products.Color&val1="Armory"+"Clear"&col2=Periods."Month"&op2=gt&val2="05/01/1998%2012:00:00%20AM""
Note that 2 features are actually enabled by the change to instanceconfig.xml above. The second is the ability to create a Bookmark Link that provides a mechanism whereby a user may obtain a shortcut link to a Dashboard page, which contains all aspects of page state. This link can then be e-mailed to another user who (assuming they had identical permissions and access) can enter that link into the URL line of a browser and view the same page content. The recipient could then further analyze the data in the Dashboard. Technically, page state is too complex to capture in a URL so this functionality writes an object to the Presentation Services catalog, which is called by the Bookmark Link URL.
Basically, the one provides a static link to the content as at creation time (with a link that is aged out based on the days set in the instanceconfig.xml) , and the other provides a dynamic link to the content based o the parameters passed in the URL.