What if you were to write a webservice method (or maybe even use an existing
one) to return the *true* max  ID?

Although I have no idea how I would call a web
service from within the InfoPath form.

This could, however, still result in problems if two users were to fill out
forms at the same time. They would both be given the same MAX ID. If I set this
after the form  has been saved there is no chance of a duplicate.

Have you considered using an event handler for this?

I would have thought that an Event Handler will require the same sort of update
as I will be performing in a workflow. Both are updating the SharePoint ListItem
object.

I guess I could do it with the Inserting Event and try to modify  the name prior
to the save. I can then still use the workflow  to modify any fields if required
later in the process.

Thought I'd put up the solution that I finally settled on.
The Event Handler (ItemAdding) did not work. I was not able to set the name of
the document  here. After some digging I came across a way to set the name in the
'ItemAdded' event, which could also be used within the workflow  so turns out I
found exactly what I wanted.
Here is the code:

string url = file.Url.ToString();

string newurl = url.Replace("XXX", formID);

//DisableEventFiring(); //Only required for the Event Handler
file.MoveTo(newurl);

// Save the changes made to the SPFile object
file.Update();

//EnableEventFiring(); //Only required for the Event Handler


Pretty simple once you know how.

Hope this helps someone else

You can do with workflow  easily. Just in workflow build a dynamic string and
save as a parameter adding  the ID with whatever else you want the name to be and
then set field  in current item and select name (for use in forms) and set to
param.

how have you been able to modify  a field  in the form  through the
workflow? Promoted fields, yes, others? Any blogs or pointers? I've never
seen/heard of this.

I used this article to help me with the setting of a field.

www.bizsupportonline.net/.../set-infopath-form-field-value-share\
point-workflow.htm

There are a lot of good articles at this site relating to InfoPath.