Hello Phil,
Attachment limits are something we deal with frequently in form design. Although increasing the limits may seem like the best rout, you may be doing your users a dis-service. The larger the attachments, the slower your forms will become and could even cause your forms to stop working properly. This is because the attachment sizes actually increase the size of your form file.
The best alternative we've found is to submit the attachments to a document library, and replace the attachment with a link. This can be done with a repeating data structure to allow your users to attach as many files as they need.
This method uses the qRules SaveToSharePoint command. Here is a link to some of the details for using the command, http://www.infopathdev.com/files/folders/qrules/entry40104.aspx
and a link to details about qRules http://www.qdabra.com/en/products/qRules_new.aspx
Hope that helps!
Bre