I have a trek with a zr aluminium frame and have been in an accident. The frame looks ok, i.e. naked eye, but a mechanical test at the bike shop revealed that it was slightly bent. The seat post was also bent, but I could not tell this just by looking at it. What are the risks of riding around with a bike with a bent frame; is it definately something that i should replace?

yes

What did the bike shop that did the test on it recommend????

The bike shop recommendd I buy a replcacement frame. I thought that was fair enough until i realised that I didn't know what was so dangerous about marginally bent aluminium frames, I was just wondering if anyone had any technical knowledge about it.

Al failure is sudden and ugly. Kinda like CF without the splinters. Let's just say that Al is not a metal that likes to be bent.

You know what, riding with the thought that the frame could suddenly fail is going to haunt you and lessen your enjoyment anyway. I tend to agree with pseduobrit.

Being a long timer, I don't know a lot about aluminum, compared to steel bikes that I have seen "realigned" or lug work done on.

Personally I wouldn't take the chance on AL.

Al failure is sudden and ugly. Kinda like CF without the splinters. Let's just say that Al is not a metal that likes to be bent.

Bending annealed Al is fine.
Bending heat treated Al is not.
Especially 7005 Al (very brittle).
If/when you decide to replace the frame and if you choose to stay with AL buy one made from 6000 series AL.

Enjoy

You are only likely to get catastrophic failure if you try to bend it back strait. The bending will have caused work hardening and any bending back will cause it to crack. So I would order a new frame and carry on riding it until the new frame comes in. The handling may be affected and you may get uneven tire wear, but excepting big jumps you can use it for transportation.

Do not pass GO
Do not collect $200
Go directly to replace the frame.

Aluminum fails without warning, and it ain't pretty when it does