IPhone 3Gs out of space and bricked

Question

Two weeks ago I did a sync of my iPhone 3Gs (28 GB), following an upgrade to version 5.1 of the software.  At the end, I noticed in iTunes that the capacity bar was solid yellow, without the normal demarcation of space by category (music, photos, etc.).  It was almost completely full, with just a few MB left.  I didn't have time to deal with it then. 
Over the last two weeks, my phone has been unbearably slow and unresponsive for functions that have nothing to do with AT&T's horrid network.  The touchscreen is very unresponsive, some apps have stopped working, functions that used to take 1-2 seconds take 5-10 or longer.  My theory is that with so little free space on the phone, the o/s struggles to do some basic functions.
So tonight I removed 1-2 GB of music and photos, with the thought of resyncing to free up disk space.  However, the "sync" button in iTunes was disabled, and I couldn't find any way to force a sync.  So I chose to restore from the backup of my last sync on March 11th.  The sync completed, but now my phone is a black screen with a white apple logo, and below it an empty bar (looks like a status bar).  But nothing is happening.  I've tried to power it off, but can't. 
My phone is now a brick.
Any suggestions on how to recover my phone, and also any tips on the optimal free disk space to ensure proper functioning would be greatly appreciate.
P.S.  At&T cell coverage in downtown Chicago has always been bad, but in the past two weeks it's been downright unusable.  I can't make a phone call longer than 3 minutes without it dropping, and forget about anything but the most mundane data usage.  I know I'm overdue to upgrade phones, but I've been holding out for the iPhone 5.  Any idea when that might arrive?

Answer 1

Follow-up.  I did the Power-Home combo to try to reboot it, but it lands at the same black screen with the empty white status bar.
I hate to restore to factory settings, but I fear there may not be a choice.