I have the same problem with my 350. I had to wipe the Slingbox because of TCP port contention with another Slingbox (Grrrrr). Here is the story:
Factory reset, good power and network lights, streaming light flashing steadily; FLASHING, not the pulsating glow of an unconfigured box and it is not showing up in the Directory on SlingPlayer Desktop OR SlingPlayer 2.0 (yes, I know I can't configure a 350 with SP 2.0, but they do normally show up in the directory). I've reset my router, removed and reapplied power to the Slingbox and repeated the factory reset procedure multiple times. Yes, UPnP is enable on my router.
What does the flashing Streaming light mean? Do I now have a brick? ...sure seem like it.
Sling? Please help us!!!!!!!!!!!
I reset my Slingbox 350 because I could not "watch" it on my computer. After resetting it, the process went fine until the streaming light kept blinking. I have already unplugged the internet connection and followed all the instructions. There is a clean internet connection, but the "streaming" continues to blink. Any suggestions as to my next step? And yes, this has worked well for quite some time.