It may be worth clarifying the relationships between the standards and open source components of Intalio. From my perspective as an architect :
The central feature of the Intalio run-time is a business process engine implementation of BPEL 2.0.
Human interaction with the business process is termed workflow and in Intalio is an implementation of loose standard specification BPEL4PEOPLE. This is packaged as an application TEMPO. There is a form-based user interface for this workflow which relies on the XFORMS W3C standard. Note that XFORMS is a device-independent standard which does not define the presentation form, only the function. ORBEON is used to present the user interface in HTML. This is a server-based implementation of XFORMS with the transformation from XFORMS to HTML taking place at the server.
The Intalio Designer component for forms design is specific to workflow and not a generic user interface designer nor even a full-function graphical XFORMS designer.
An advantage of the open source approach is that components can be bypassed or supplemented to extend the capabilities.
If your organisation has adopted XFORMS as a standard, it is likely that the limitations of the Intalio Designer will force you into a development method that delivers the XFORMS by other means. You could use the Intalio ORBEON implementation for your separately developed XFORMS or use the browser implementations of XFORMS (eg XFORMS Plugin in Mozilla Firefox or Formsplayer for IE). Designing a user interface with XFORMS is not necessarily a job for a graphical design tool. Unfortunately, the Intalio Designer does not provide a source editor for the XFORMS XML and may loose edits that you make externally to forms.
There is a reasonable application design/development approach in which a business analyst uses Intalio Designer with the included workflow to develop a working model of the business process. Then the user interface could be replaced with a house style anywhere in the range from all singing and dancing Silverlight down to plain text forms.
Developers asking how to add buttons or improve the look and feel of Intalio forms for production quality applications would be advised to look at their forms as a separate piece of design rather than attempting to work within the constraints of the Intalio implementation. The interface between the presentation layer and the TEMPO and BPMS components is fully defined and implemented in standards.