ah - thought this thread was going to die without trace :-)

quick update - the "udhcpc -fnq -i tap0" command does, as I said, result in being able to browse the net, but I'm not happy about it and I wouldn't reccomend it as an approach (at least until someone more net-admin savvy can confirm it's not risky ... I really don't understand well if there are any security implications to it grabbing a new ip from my router/DHCP server when it already has one form the VPN pool).

Meantime, I've come up with another bodge - I've setup a proxy server (freeProxy) on the same XP Box that hosts my openVPN. Once I'm connected to my VPN from the N900, I open a browser window (which fails, as expected) and then I just set it to use the local proxy server 192.168.0.10 using about:config.

As examples:
network.proxy.http=192.168.0.10
network.proxy.http.port=995
network.proxy.ssl=192.168.0.10
network.proxy.ssl.port=995
(the above can be set from settings->internet connections->connections->(select your connection)->edit etc. and they will be retained even if the proxy is disabled)
network.proxy.type=1 (default=0 for no proxy)

Once you enable the proxy, you can still access all addresses on the local LAN OK and you can browse the internet too via the proxy. It's another crap solution (and in fact little better than just setting the browser to use my proxy via my router's public IP, port forwarded to my XP box, with no need of VPN at all), but it may be of some use in the scenario whre you need to be on the VPN for access to internal servers but simultaneously need an internet connection.

Note #1 - the network.proxy.type value is NOT persistant - i.e. it resets back to zero every time a new browser window is opened. Not sure why, just another bit of random cr@p to befuddle me during testing I guess :-)

Note #2: All my testing has been done at home, over my gprs connection (haven't got around to trying it from a public wifi yet even though that was the point of me setting it up) - From other stuff I've read here, there may be different/less/no issues getting net access over VPN if connecting via WiFi so please check before changing anything on your work LAN ...

Please report back on your own success (or otherwise), and avoid my clumsy hacks if at all possible :-)