Today, I went into Cydia, and tried updating an app I downloaded. When it was done downloading, it said to restart SpringBoard. My phone then crashed, and has been in an endless reboot loop for 2 hours. It shows the apple logo, then a loading symbol, then the screen turns black for a few seconds and restarts again.
Is there any way to fix this without restoring?
Thanks.

It did this to me when I installed the Rotate Inhibitor for SBSettings.

I held Power & Home until it shut off, then turned it on. I had to do that a few times and eventually the phone started in safe mode, which then allowed me to go into cydia and uninstall the app.

Hope this helps

But sometimes, this doesn't work.. then your only option is RESTORE-it..
It happens to my 3G iphone before...

So after trying couple of times hold power and home, re-jailbreak, but it still keep doing endless reboot/loop, so I just RESTORE-it

Put it in Recovery Mode, Plug in to your computer, iTunes, Shift+Restore, yup.. Sadly..

but at least it works...

I never try that at 3GS, but i believe it gonna works too.

It works...

Been there, tried, it... bought the t-shirt.

~Adrian