Emergency UNIX and Linux Support

And you're both stuck in arguing about a miniscule detail completely unrelated to the OP's question...

@hiten.r.chauhan: How is the NIC configured for the guest? Options are usually NAT, Bridged adapter, Internal Network or Host-only adapter. If it's bridged, make sure it's bridged to the correct NIC on your host machine. Also, can you post the contents of /etc/resolv.conf and /etc/defaultrouter of the Solaris guest?

Neither is it a minuscule detail as blindingly following shelladdict advice would likely break the OP virtual machine nor is it unrelated as the guest tools associated with a VM might help fixing some issues although not in the OP case.
As I already wrote, using NAT and DHCP is the simplest way to get it working.

First: trying to install VMware Tools in a VirtualBox machine (and vice versa) won't work (I've tried), aside from the fact that it would take a lot of intentional tinkering to even attempt it. So it's pretty unlikely that this has screwed up the system

Second: yes, NAT + DHCP is the most simple approach, but only if both the machine and the system within are configured to use that, which is what I'm trying to find out. There've been questions like this before where the problem was a simple misconfiguration (Static IP used when DHCP should have been used and vice versa), so my approach is to get that out of the way first.

NAT + DHCP work well on a Solaris 10 guest in VirtualBox on Windows 7. First check that VirtualBox's DHCP server is enabled (File Preferences Network) and NAT is the selected for the guest network adopter.

If you run into network configuration issues in your Solaris guest and you are not familiar with Solaris's networking files, simply do /usr/sbin/sys_unconfig to unconfigure everything and reconfigure it on reboot. Ensure that you select DHCP and DNS for your networking options.

NAT + DHCP will work best ...If you don't want dhcp....you can sys-unconfig and make it to static...internet still will work fine..

you can choose "bridge" if it's possible