changing the sid will have no effect on the text message issue,in fact just to change the sid with plist editor so that it reads whatever your changing it to,wont actually change the sid,you see the phone will ignore those changes and continue to use the pre-installed sid.

as of right now there is no fix for the text message issue,you can only improve it (a little). using a custom ipsw made with snowbreeze for 4.2.6 there will be a slight improvement,i mean slight.after i loaded it i noticed i was able to get text from my wifes phone but still could not send to her,she is t-mobile.now this is gonna sound wierd but i also have tmobile and can not send or recieve to / from my phone to iphone 4,and that doesnt make much sense,because they are both t-mobile. i put up a custom jailbroken ipsw that i cooked in the iphone forum here at gurus if anyone is interested,but read all the warnings on that post before loading,as of now you will not be able to go back to a stock ipsw after loading it.