It seems like text message evidentiary issues come up most often in criminal cases.  In a civil case, sometimes people battle over whether they get to inspect the PDA of one of the other parties, but I'm not sure how much of this is posturing and how often people just rely on the other side to produce relevant emails and communications.  (This would be worth taking a close look at - how often have courts approved "inspections" by a party of the other party's computer and what conditions have been attached.)  Either way, outside of divorce or criminal proceedings you don't seem to see many text messages evidence issues.

A recent Kansas Supreme Court decision (access here [pdf]) squarely addressed some text message evidence issues that highlight the main things to consider when seeking to admit email or SMS.  In this case, someone was charged with discharging a weapon (shooting at someone's house).  The defendant supposedly sent some text messages after the incident to a third party.  The police talked to the third party who showed the cops the text messages (they were copied down verbatim).  At trial the prosecution did not produce the actual messages, they produced the police officer's recordation.  This evidence was admitted over the defendant's objection and he was now appealing this decision.

He raised a few arguments, all of which were rejected by the court:  (1) best evidence rule, (2) authentication, and (3) hearsay. 

On the best evidence issue, the court noted some ambiguity on whether the rule applied, given that the "writing" was not necessarily reduced to "tangible form".  The court didn't need to resolve this issue since a statutory exception applied - the person who received the messages had lost his phone. 

On the authentication issue the court found that the testimony from the witness that he owned the phone, had programmed the phone with the defendant's name and number, and had received the messages from this name and number was sufficient.

Finally, on the hearsay issue the court ruled that the evidence was admissible as a party admission.  While the parties did not discuss the excited utterance exception, I'm guessing the circumstances would have warranted this exception as well.

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Bottom line:  this type of evidence is admissible in the same way any other evidence is admissible.  It's a mistake to assume that these types of messages are self authenticating and to not provide any other foundation.  On the other hand, it's not necessarily true that you need a copy of the message produced by a party who sent or received it.