Fire up the browser, open a page, and after the rendering is complete, call the N900 from another phone. It should ring.

According to Forum Nokia, the N900 is an EDGE Class A device. This means it should be able to make phone call and have a data connection at the same time. (http://www.forum.nokia.com/devices/N900/ and http://cat.inist.fr/?aModele=afficheN&cpsidt=947489)

So is this a bug?

Ok, i'll give it another try, this time with visuals.
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^-these are the channels your phone uses for both data and voice on 2G/2.5G
these channels either transmit data OR voice. Imagine, if ALL the channels are transmitting data, there are no channels left for voice, so no matter what kind of a device you have, it won't ring. IF there are free channels available, THEN those can be used for incoming calls.
This was always the same since GPRS was born.

Incoming calls are not signaled over the same channel as data or voice

And EDGE Class A devices CAN do both, always.

i recieve calls while browsing.

Thanks, i didn't know that, however, i have not had any phone that didn't suffer from this (and i've had a few from various artists). Perhaps he was on GPRS not EDGE?
edit
andybehr: i just looked around, and found out, that calls and data together depends also on the network, not just the phone. See http://reviews.cnet.com/8301-19512_7...ag=mncol;title and don't mind that it's about the iphone, that's class a too