I have a non-retina mid-2012 Macbook Pro running Snow Leopard and this issue has been plaguing me for a while.
During regular use of the laptop, while connected to my home WiFi network (to which no more than three other devices are usually connected), I keep experiencing sudden and complete drops in connectivity. It happens more than once every hour, usually while I'm browsing the Internet on Chrome and I'll see my loading tabs hang there with no response. The OS does not seem to notice this drop in connectivity, and keeps showing the WiFi icon as connected. To get the connection back I either have to wait till it returns (usually after a minute) or disconnect and reconnect the WiFi.
I think the issue is with the Macbook itself or between the Macbook and the router, because during these drops I won't even be able to connect to the router itself (by opening its web interface), and the Macbook seems to completely ignore the lack of connectivity.
The other laptop in the house (running Windows) does not experience these connectivity drops.
I have tried resetting the router, making the Macbook forget about it and then re-configuring the connection and changing WiFi modes but none of these things worked.
Here are a few more details:
Model: Linksys WAG120N;
Firmware version: V1.00.16 (most up to date);
WiFi mode: Mixed (running in G-only or N-only mode still causes issues);
15" Macbook Pro from mid-2012 (non-retina);
Processor: Intel Core i7 2.6GHz;
Software: OS X 10.8.2;