Someone on the bug for this logged on Launchpad has found a PPA with the new client in :-)

https://launchpad.net/~turl/+archive/ppa

Saves having to download the other package and do the messy moving around of files. Have just tested it on mine and I'm not getting the bad signature error any longer!!

There is a bug filed for this issue on Launchpad:

https://bugs.launchpad.net/ubuntu/+source/boinc/+bug/585547

No sign of a workaround on there but at least it's been noted and a possible solution via Synaptic *may* become available :-)

Getting same issue on a couple of freshly installed Ubuntu Lucid machines.

Oddly I have Boinc installed on a couple of other machines running Lucid that where upgraded from Karmic and they are working fine so it does appear to be an issue only on fresh installations from my cases.