"Some" is probably an understatement. I'd wager most of those who use their N900 for FM radio, web radio, or local music playback have encountered this on multiple occasions.
It happens very frequently for me. I guess renicing apps might help but i haven't tried that. Basically, whenever i am using N900 while music is playing, i expect to have a few blips or stutters. It doesn't usually happen if it's just in my pocket.
I've only had this issue when I get something that spikes the processor, or tries to play a compeating sound, while listing to audio. A good example would be an incoming MMS message, or gPodder doing a poorly timed sync/update. I have the same issue on my computer at work though, so it's not surprising. Any time you heavily load a system doing audio playback you have a chance of causing skipping.
I've read some threads (search is your friend!) here that people have pretty much eliminated this by renicing some of the processes around playback and/or by overclocking. It just hasn't been worth the effort for me to bother with, since it doesn't happen often, and isn't that annoying to me when it does.
Oh: And I've never had it happen during a call with call audio. I've only ever had it happen with audio playback from the device (mp3s, etc).
There are two kinds of stuttering. The first happens with non-Nokia applications when, e.g., locking the screen or rotating the phone. Nokia thought it would be very clever to actually freeze all applications it doesn't trust (i.e. all except those made by Nokia and explicitly whitelisted in the OS) to ensure smooth transitions. It turned out to be not so clever after all, esp. since the GStreamer sound pipe is prone to die during freezing (causing e.g. FM-Radio to sometimes not playing anything after thawing, until you pull and reinsert the headphones to force a restart of the GStreamer pipe).
The second kind of stuttering affects all applications. The pulseaudio sound system is a resource hog, and along with other resource hogs that kick in from time to time without being asked for (like hildon-thumbnailerd) it causes stuttering. Renicing pulseaudio to a low level below zero helps to relieve this a bit.
Nokia should've done their research with renicing and kernel parameters, but obviously they haven't.
Maybe Apple and Steve Jobs are just right in this issue. Multitasking is not suitable for mobile devices with these current hardware resources - still.
I had serious problems during this week trying to just use Mappero and µBrowser at the same time during visiting a foreign city abroad. (PR 1.2)
Yet trying to take photos while those two are in the background, will make N900 unresponsible and a few times I had to reboot by removing the battery. It is irritating!
N900 does not have enough RAM-memory perhaps, or kernel is jut not suitable. It shouldn't (thinking it is a Linux system) go so dead just multitasking 3 applications!
Nokia Maps was useless, as it cannot route or find locations offline and even if routes and locations are found when connected to WLAN, one cannot save them anyway, so you will probably loose them when going outside the WLAN zone. Mappero would work, but it is perhaps too big program to be usable in N900 - one wants to take photos also while tracking.
I must say, I was very dissapointed during this week. Should have taken my old N95 with me instead.
I have the same issue on my computer at work though, so it's not surprising. Any time you heavily load a system doing audio playback you have a chance of causing skipping.