I-540 appears 1997 Official

I-540 extended to Leesville Rd 2000 Official

I-540 extended to Falls of Neuse Rd 2002 Official

In 2003, I-540 was extended east to US 1.
In 2004, I-540 was extended east to Triangle Square Blvd.
In July 2007 (per Adam Prince blog post), the segment of soon-to-open freeway from I-40 southwest to NC 55 was renumbered from I-540 to NC 540 due to lobbying from the NC Turnpike Authority.
In Dec 2008 (NCDOT), I-540 was extended from US 1 southeast to US 64/264 (now also I-495). This was shown as complete starting with the 2007 official.

I-540 extended to US 1 2003 Official

I-540 extended to US 64-264 2013 Official

Exit List:
EXIT 1: I-40, NC 540
EXIT 2: SR 1002 Aviation Pkwy
EXIT 3: Lumley Rd
EXIT 4: US 70
EXIT 7: SR 1839 Leesville Rd
EXIT 9: NC 50
EXIT 11: SR 1005 Six Forks Rd
EXIT 14: SR 2000 Falls of Neuse Rd
EXIT 16: US 1
EXIT 17: Triangle Town Blvd
EXIT 18: US 401
EXIT 20: SR 2215 Buffaloe Rd
EXIT 24: US 64 Bus
EXIT 26: I-495, US 64, US 264

All set to be more of I-540 (Adam Prince)

Actually opened as NC 540 (Bob Malme)

If I-540 is ever extended southeast back around to I-40 in the Garner area, will it be renumbered as I-640? This was the plan from 1996-2002 when NCDOT changed its position on this question. For now 540 is here to stay.