I had same issue updated to IOS 7.0.3 last night on my Iphone 5 for Sprint and same thing Invalid SIM came up.
So I went to Sprint Store thsi morning and they cleared and uodated eveythign on the phone. Nothing. Was told to go to Appl eStore.
Went to Applt Store, they checked the phone and all was good. They even gave me a new phone. Same thing Invalid Sim.
Went to another Sprint Store. They put a NEW SIM card in my NEW Iphone Apple Gave me and same thing. Works fro liek 5-10 minutes then the error messag epops up.
No one seems to know the fix. Tried all the BS on here. All the ##clear# , ##update# and nothing.
Evwyone wants to blame everyoen else but the customer is geeting screwed.
Only way to gt ti to work is takign out the sim and puttign back in again and it will work for 10 minutes or so so if u need to make a call thats it.
Otherwise was todl by Sprint "No is NO Solution at this time"
Guess what...if u dont fix the solution would be I'm gonna port over to Tmobile. Oh and Apple u too, you released the update that caused this mess so get your act together or I'm going Android.