I noticed that sometimes the sound is corrupted when I receive skype calls, the sound is scrambled and it's impossible to understand. when I call back the sound is ok again. Has anybody else observed the same problem ?

No, but I've not done mnany calls yet. It sounds like the two endpoints of the Skype converstation might be "agreeing" on and using different encryption settings. Look to see if there's a pattern, like N900->Mac Skype v 72.1 bad, N900->N95 OK, and report it. Keeping a paper record is generally much more likely to spot this sort of thing.

I do find that the first few seconds of the call is frequently scratchy, but that it sorts itself out. This is N900->Win7 Skype 4.?, as a datapoint.

Actually it happens when receiving calls through the associated US number. I bought a US number for skype because both at work and home have wireless around and that way instead of paying $60 for a contract with AT&T or t-mobile I simply pay $3 with skype for ilimited calls (I use a backup pay-as-you-go at&t sim card for the occasions in which I'm not in a wireless range, in which case my skype number will forward the call to the sim card number). The only problem I'm finding with this scheme is the one mentioned before (and it's saving me loads of money!)