Rebooting iPhone Doesn't Kill off All the Process

The only real way to be certain that you have killed apps off is to jailbreak.

Jailbreaking lets you install a piece of software called SBSettings, which includes a real task manager that lets you force-quit any app, including the Apple apps.

Jailbreaking is awesome for lots of other reasons too!
I got this picture off my iPad by starting the Samba daemon I have installed, and just browsing to the image using windows explorer.
No cable or iTunes needed,

As a developer I can assure you that if you reboot the phone, those apps are no longer running in any way. At that point it's literally just a list of apps that you've launched in the past in reverse chronological order.

Edited to note: I've since verified (as @ughoavgfhw pointed out) that backgrounded apps can in fact have their services restarted after a reboot. You can still kill all non-Apple services using the method I describe below.

If you still don't feel comfortable, you can manually quit the apps any time. Simply double-press the home button to reveal the list of "running" apps -- the multitasking bar -- and tap-and-hold on one of them, the same way you rearrange you apps in Springboard.

They'll begin to wiggle, and instead of the black circle with an "x" in it that appears in the upper-left corner of each app icon (used to delete apps off the device entirely) you'll see a red circle with a "-" in it. Tap the red circle and the app will be completely forced from memory, killing an associated processes. After you've quit the first several, even if you haven't restarted your phone you're in the "totally suspended" territory.

Press the home button again to stop the wiggling, as normal.

Deleting an App: Black Circle with "x"

Quitting an App: Red Circle with "–"

Empty multitasking bar, all apps have been dumped