For text messages, sigs are a little weird. I mean, why? text is supposed to be informal, quick communication.
In regard to emails, my sig is, "Sent from my wireless device". This has been my sig for my Palm, my WinMo, and all the other phones I had. I don't find it necessary--especially my professional contacts--for people to know what iteration of what device I'm using. If they want to know that, they can look at facebook and see the "posted from my Android device" thingie that shows up next to my wall updates.
I'm right in between those generations where email--or any written correspondence--is a semi-formal excercise with an introduction, body, and sign-off sections, and the other iGeneration that write me emails with "lol cu later" that is "Sent from my iMustHavePhone". So, I feel it's courtesy, at least for old curmudgeons in their 30s like me, to announce that any terse or impolite seeming email is probably due to the fact it is "Sent from my wireless device" I feel it's a little obnoxious to use it as an epeen feature and broadcast some propaganda about the device I'm using.