Ditto to the comments from Mulligans Missus. If you cannot connect to your wireless access point then I would say that you have cause to return your Mac, or at least take it in to the Apple store to get checked.
But if you can connect to your wireless access point and after a period of time, say after waking the Mac from sleep, you lose your wifi connection then this does not guarantee your Mac is faulty and more likely to be an OS issue. Based on the number of users in this forum complaining about losing wifi connections while using Lion, it would appear that there is a problem with this new OS, especially when you compare to previous versions of OS X. But that figure still doesn't mean there is a common fault with Lion and wifi. My i7 MBPro running 10.7.2 has no issue at home when connected to my Airport Extreme and at work when connected to a Netgear wireless router. And we have 8 Mac's (iMac, Mini and MBPs) running 10.7.2 in the office that also appear to work fine on wifi at work and at home - at least they are not complaining to me about wifi issues.
So take it in and get it looked at while it is still under warranty. At least you can get a another opinion about how well it is/isn't working.