Mine still works well with a collection of iphones and ipads; there is only one version of
AP-800 hardware and I'm using hwatever firmware it came with, so I wonder what
the different factors are.

Ok, I retract some of my slightly ranting post.
I have re-tested with new AP-800's - FW1.02 (but thinking of loading 1.03 from global site) and all is Ok I think and hope....so i presume a hardware fault with one of the units in the mix.
I have a few more AP-800's to deploy in next few weeks so will report back.
I now have other issues with WDS but that is another story.

I should also add we force the router to use specific DNS server address's and pass this to clients using DHCP if you don't we tend to see loads of issues with Mac's and often have to make sure with manual DNS settings in Network settings.
I suspect other peoples issues are related to Apple's poor implementation of DNS resolution. They have messed too much with the very solid *BSD foundations and left us sysadmins a turkey shoot out with their IPv6/mDNSresponder implementation.

Also for Mac's we always turn off IPv6 on network interfaces but when our networks move to IPv6 fully in next year or so we will more than likely have to turn it all on. BTW. networksetup command is your friend on this one.
eg. networksetup -setv6off Airport ( Now sensibly called Wi-Fi on Lion ) or networksetup -setv6off Ethernet

I'm only using ipads/phones/pods not macs and didn't sent anything specifically on DNS so I assume it's all DHCP, dafult and using Vigor router as dns proxy.

It's not a DNS issue. Though certainly Macs do have DNS oddities, particularly when moving between networks, this is something else.

The iDevices drop off the wireless network or they show that they can't get a DHCP address. Reconnecting them to the wifi works for a random period.

Also, swapping out the AP800 devices with netgear or cisco and you never get a problem at all.

For what its worth, I'm running AP800 with 1.0.3 firmware, and Macs with Lion wifi, and Win 7 wifi, and iPhones with iOS 5, without any problems. The Ap800 is providing both 'clean' and 'dirty' internet access (via separate SSIDs). So i seem to be able to provide 'all of the colours in all of the sizes' to my users, so it can work. PS - after 3 months, i notice that the UK 'support' website is still offering Brits only 1.0.2 firmware for the AP800, whereas a few people on here (like me) have got the 1.0.3 f/w from the global draytek.com website, and I can recommend it as being fit for UK operation (well, England & Wales anyhoo - Scots may wish to set out their own timetable for the availability of v1.0.scot on the ground that it will be their right to download that version simply because it is not English & Welsh).