You can use the Web to X forms to designate a specific record type for each of the records submitted through your forms.
By inserting the ID of the record type where you find the Xs below, the record type will be automatically be selected upon creation of the record.
<input type=hidden name="recordType" id="recordType" value="XXXXXXXXXXXXXXX">
This will select the record type when the new record is created and will override the default record type of the user it is assigned to.
*Note: The record type on the submitted form make sure that the option "Keep the existing record type" is selected as your Record Type Setting:
Setup | Customize | Leads | Lead Settings - For leads
Setup | Customize | Cases | Support Settings - For cases.