When a potential client signs a form, would a fax from his fax machine, of the form, carry an legal weight, if their were a dispute? I understand an actual mailed copy would be best, but would a signed, dated, faxed copy from the signers' fax machine show any legal understanding? How about a scanned, attached to email, copy?
Check out Public Law 106–229 TITLE I—ELECTRONIC RECORDS AND SIGNATURES IN COMMERCE. Basically it says an electronic signature is valid to enforce a contract. However, there is still a lot of confusion in the state and federal courts about how to indicate your acceptance (i.e. browser wraps vs. click wraps). A clear communication that purports to accept a written agreement is going to be enforceable.
In the UK the original, faxed and scanned documents can be submitted as evidence. Given it sounds like you are taking about contract law and therefore civil, rather than criminal, the judgement is based on the balance of probability and so if you provide a scanned document and the other side claim it has been altered it is up to the judge to decide who he believes basically. This is exactly the same as if it were a physical original document