Here is the video you all been waiting for!
I have my older 3GS (if 3GS works so should 4 and 4S, which I left at work - oops!) sitting here in standby, for about a few hours without being plugged in or touched or turned on (just standby), and then I go onto my laptop, start up iTunes, wait for my iphone to show (takes a few secs to a minute depending on your network), it then pops up, does its startup/pre-connection circle thingy until it connects and shows the Eject button, then I press the sync button, it'll search for my iPhone, connect and start syncing, all from within my iTunes on my laptop...
Also check your virus programs, firewalls, and network connections from blocking any incoming/outgoing connections to your iPhone/iTouch!
5 other friends of mine do the same thing, this all works for them too! 2 have the AVIC-930BT and a iTouch 4G, so they love this leaving in the vehicle, the other 3 just have it hooked up as an AUX input to there deck, but don't have to bring them in anymore!
I hope you guys that are having problems can solve the issues you are having, maybe firewall, network/router settings? I can access my phone or touch at ANY time, there is no timed-out or anything, unless your router's wifi settings or some other setting is causing it to disconnect (maybe some type of idle mode/no activity autodisconnect option).
Because when it works, its so great and convenient! It was always a PAIN to remember to bring in the iTouch or forget the iPhone in the car all the time! I usually leave my iphone in my truck connected for netflix and never really use my phone in the house often anyways, just take it out during the day while im out and about and when I'm away from the truck, but leave it in the truck overnight, except tonight I left my 4S at work! So my old 3GS is shown here doing what I mentioned! I'll post a video of my deck retaining power which is not necessary or required anyways for alot of ppl since you can SYNC regardless!
If still no go, try this,
Open the windows firewall settings, and look if "SyncServer.exe" is allowed. I'm gonna guess it isn't even listed 
Instructions (Windows 7 x64 - Vista is probably similar):
Open "Control Panel"
Select "Windows Firewall"
Click "Allow a program or feature through Windows Firewall"
Look for "SyncServer" in the list. It won't be there.
Click "Add program" or "Change Settings" then "Allow another program"
Click "Browse..."
Find "C:\Program Files (x86)\Common Files\Apple\Mobile Device Support\SyncServer.exe"
Restart iTunes and your iPod/Phone/Pad should appear in iTunes even when disconnected. If not, try rebooting to make sure everything is reloaded properly.
Also you can try to reboot your device too, don't just put it to sleep, actually power it off fully and power it back on... that seems to solve alot of issues such as 3G connection, WIFI connection, etc.