Vacuum in Petrol tank

Mine has postive pressure and I think that's normal. (5.2 single fuel pump model). I've never heard of a vacuum - and it must make unscrewing the cap even more difficult than it is already.
As for the cause of your problem, there will hopefully be some experts chiming in. To start the ball rolling, may I suggest a problem with the vent pipe (142104) or the anti-evaporation system (which ends with the cannister low down in the rear left hand corner of the car). In addition to the vent pipe, there are three pipes from the tank to the disareator.
Maybe the system isn't "breathing" properly, with the fuel used from the tank creating that vaccum.
Look at Daniel's (ricambi) web site to see the drawing of the pipes etc coming out of the tank. Type a part number (e.g. 142104) to get access to the schematic - Table 11
Hope you get a fix soon.

 

Well I'm no expert but my car does the same thing as listed above (poss. pressure) '95 2.7, I'm pretty sure if you do a search you will find a few posts on this. If I recall someone could'nt even get there cap off after putting it on a little to tight..

 

There's a well-known case of Ernie having a malfunction (valve?) in the charcoal emissions system of his 348, resulting in a vacuum in his fuel tank, and partially crushing it! I'm assuming the 355 is similar, although don't know. I'd recommend you have your system looked at soon!

 

This is from page 4 of 14 in the recent USA Recall Campaign 48 (for the 355 fuel block replacement). It would SEEM to indicate that pressure is to be expected rather than vacuum (can you ever be 100% certain with Ferrari documentation??) :

"Unscrew and remove the fuel filler
cap to release any pressure from
the supply system. Then, screw the
cap back on – (Fig. 6)."

 

What Rick said, check the charcoal canister purge valves.

Here's the how-to http://ferrarichat.com/forum/showthread.php?t=241642