I text-messaged my friend on his cell phone last night and this morning I got a reply saying, "I had saved your number, ...but I didn't check my cell until now.."
Is "didn't check until now" okay, or should it be "hadn't checked it until now" ?
Hadn't checked until now would be incorrect, as hadn't checked (or had not checked) is the past perfect tense, meaning he would be saying that something had not yet occurred at a moment in the past. However, now indicates that the point of reference is the present.
I think your primary confusion is the change in tense from had saved (past perfect) to didn't check (simple past). I would actually take issue with the first part and change it to the simple past to match the second half:
I saved your number, ...but didn't check my cell until now
Though I personally would suggest a simple past followed by a present perfect:
I saved your number, ...but haven't checked my cell until now.
But keep in mind that grammar (and spelling... and in some cases coherence) are rarely considered mandatory in text messages.
Didn't check would mean, an activity, checking, was not done. Whether they check always or not.
Hadn't checked would mean, possession of, checking activity. Which leads to the meaning, he/she would always check there sth(something, cell) but on this occasion it was not done.
"Didn't check until now" has the meaning that his cell wasn't checked up until now.
"Hadn't checked until now" had the meaning that he was going to check it but did not have the chance until now.
So it depends whether your friend was delayed or not.