@ananan01
Yes, I have Apple Care which to be honest if I didn't have I'm not sure whether Apple would have replaced my wifi card and motherboard. They did say that this wouldn't include fixing the wifi ariel as this would require replacing the retina screen as it's attached to the side of it.
The first time I went to the Genius bar I took a screenshot of my wifi network using Airport Utility and some stats from running Wifi diagnostics (which you can access by pressing the <alt> key while clicking the wifi icon in your menu bar) which saved an output file on my desktop. On this first visit the Genius advised that I had quite a complex wifi set-up (Airport Extreme with 3 airport expresses extending wifi coverage) so I should try rebuilding the network as well as creating a new admin user.
So I did all these things but the issue reoccured, so this time I called Apple Care directly and while on the phone we were able to reproduce the issue logged in as a new admin user. I then booked a 2nd session to take my laptop in to the Genius bar and this time to be safe I also took some additional screen shots using the Airport Utility from the Wifi scan option (to prove there was no interference from other networks) and the performance option to prove there was no interference from other networks. You can do the same as follows:
1. Hold down the "option" key and select the wireless icon from the main menu bar.
2. Choose the "Open Wireless Diagnostics" menu item.
3. After entering your computer's password, you will see the "Wireless Diagnostics" window with the "Introduction" paragraph.
4. You may ignore the "Continue" button.
5. From the main menu, choose Window -> Utilities.
6. You may optionally close the "Wireless Diagnostics" menu at this time if you choose.
7. In the "Utilities" window, select the "Wifi Scan" and "Performance" tabs for a real-time view of your network performance
On my second visit to Genius we couldn't reproduce the problem, my machine connected fine to their network and the one outside in the shopping mall but they agreed to replace the wifi card and motherboard on a kind of 'hope for the best' basis. Since I've got my laptop back I've had no more issues.
Hope this helps
VP