West Bromwich stay in the bottom three, but they gave their self-belief a huge boost with their first point under new boss Bryan Robson.
The PSV Eindhoven scouts will have taken heart from Arsenal's uncharacteristic lack of penetration against a determined defence.
Only a howler by West Bromwich keeper Hoult, who fumbled Robert Pires' 54th-minute shot, handed Arsenal the lead.
The Gunners also failed to keep a clean sheet for the 10th successive game - a worrying statistic for a defence once among the meanest in Europe.
In the absence of the injured Sol Campbell, they look hugely vulnerable against crosses and set pieces.
Earnshaw punished them from a cross on this occasion, just as Southampton and Tottenham exposed the Arsenal defence in recent games.
Gunners boss Arsene Wenger is confident that Campbell will agree a new contract soon. The England defender's long-term presence is vital to the Gunners.
Meanwhile, Arsenal face an injury crisis in the centre of midfield. Patrick Vieira and teenager Cesc Fabregas are the only two fit players available.
Gilberto has been sent home to Brazil because of a serious back injury which threatens his season. His compatriot Edu is out for at least another fortnight with a broken toe. Former Marseille midfielder Mathieu Flamini is sidelined with a knee problem.
With hindsight, Wenger may now regret letting the experienced Ray Parlour join Middlesbrough during the summer.
This was Robson's second game in charge of West Bromwich. His first was the 2-1 defeat against Middlesbrough last Sunday when ex-Arsenal striker Kanu missed a sitter.
Kanu overcame a groin strain to start against his old colleagues. He was the focal point for Albion's clearances from the back, but he proved an ineffective lone target man.
Robson explained: "I thought Kanu would hold the ball up. I wanted five in midfield too, so I had to make sacrifices in attack. But Rob (Earnshaw) came off the bench and earned us a point."The Baggies' tactics were to congest the midfield and defend in numbers.
They allowed Arsenal to have the ball in wide positions and concentrated on sealing the middle.
Vieira headed straight at Hoult as Arsenal exerted early pressure. Andy Johnson blocked a Thierry Henry shot while Fabregas headed over from a Pires corner.
But fighting for crosses around the box is not the Arsenal's game.
West Bromwich posted a 29th minute warning too. Neil Clement whipped over a cross which Johnson failed to meet with a diving header and Zoltan Gera poked a shot into the side netting.
Arsenal stepped up the pace as half time approached. Clement made a last-ditch block to halt Vieira then robbed Freddie Ljungberg at the cost of a 38th minute corner.
Pires took the kick, Pascal Cygan's header brushed off Clement and Hoult saved Fabregas' snap shot.
The Baggies keeper parried out Henry's curling free-kick to keep his side level at the interval.
Henry's 47th minute burst from the left caused confusion in the Albion defence, but neither Pires nor Dennis Bergkamp could manage a shot on target.
Then Hoult gifted Arsenal the lead. The keeper seemed to have Pires' 25-yard curler in his grasp but he let the ball slip through his hands to trickle over the line.
The Gunners proceeded to spurn chances to kill the game.
Pires found Henry who sped past three tackles before mis-hitting a shot. Bergamp picked out the unmarked Cygan on the far post but the centre back headed over from close range. Vieira shot wide and Bergkamp had a goal wiped out for offside.
Robson sent on strikers Geoff Horsfield and Earnshaw for Kanu plus midfielder Cosmin Contra with 15 minutes left.
The move paid off within four minutes. Jonathan Greening controlled a long pass and crossed to the far post where Earnshaw nipped between two defenders to stab home the equaliser.
Albion retreated to protect their point. Arsenal sent on Jose Antonio Reyes for Fabregas. The Spanish winger soon drove a free kick over.
Vieira flicked on a Pires corner in the Gunners' last attack but Ljungberg could not apply a decisive touch.
MAN OF THE MATCH: Darren Purse (West Brom) - Gave a rock-solid performance at the heart of a well-organised defence.