OK, so I've had this problem for some time on two Mac Book Pro's.
When I put them to sleep by closing the lid, and then wake them up by opening the lid, all GUI networking stops working and doesn't work until the machine is restarted.
So tonight this annoyed me too much and I decided to investigate.
This is the diagnosis I've done so far.. it points to mDNS, surprise surprise! They are both running 10.6.8 and have had all firmware updates etc applied. Although they were manufactured about a year apart, they both have Broadcom 57765-B0 network cards.
The logs show this:
Mar 14 23:12:43 com.apple.SystemStarter: 2012-03-14 23:12:43,366 INFO [RuntimeAnalysisMonitor]  - Memory Analysis (runtime)>> Allocated: 29256 kb >> Free: 9270 kb >> Max: 126912 kb
Mar 14 23:12:43 mDNSResponder: *** process 43 exceeded 500 log message per second limit - remaining messages this second discarded ***
Mar 14 23:12:47: --- last message repeated 2 times ---
Mar 14 23:12:46 mDNSResponder: ERROR: mDNSPlatformReadTCP - recv 74 got CLOSED 435000 times
Mar 14 23:12:46 com.apple.SystemStarter: 2012-03-14 23:12:46,885 INFO [RuntimeAnalysisMonitor]  - Memory Analysis (runtime)>> Allocated: 63424 kb >> Free: 40257 kb >> Max: 63424 kb
Mar 14 23:12:46 com.apple.SystemStarter: 2012-03-14 23:12:46,978 INFO [Live Status Collector thread]  - Cleaning all expired Live Status messages
Mar 14 23:12:47 mDNSResponder: *** process 43 exceeded 500 log message per second limit - remaining messages this second discarded ***
Mar 14 23:12:54: --- last message repeated 6 times ---
Mar 14 23:12:54 iChatAgent: [Warning] IMDServiceSession <AIMServiceSession: 0x100123700>: Want to connect to slogin.oscar.aol.com, but it is NOT up
Mar 14 23:12:54 mDNSResponder: *** process 43 exceeded 500 log message per second limit - remaining messages this second discarded ***
But the following tests work:
www.bbc.co.uk canonical name = www.bbc.net.uk.
telnet 188.8.131.52 80
Connected to 184.108.40.206.
Escape character is '^]'.
HTTP/1.0 400 Bad request
<h2>Client sent a bad request.</h2>
Connection closed by foreign host.
Now.. this shows that networking is up, and we can connect to port 80 on a webserver, ie we can surf the web....
But everything in the GUI fails to connect to networking.. ARD, afp, web, mail, ical etc.
The solution is to force quit the mDNSResponder either from the command line with:
sudo killall mDNSResponder
or to select the mDNSResponder process in Activity Monitor and force quit it.
Because it's managed by launchd it will automatically relaunch, and when it does networking is instantly restored.
I haven't yet worked out why it goes wrong, but at least I know where the problem is...
Next step is to diagnose the wake up process!