CX-100 series is notorious for this.  I luckily came across a pair practically free since they came with the 4500 system I bought, and the price for the whole system with speakers was less then I was ready to pay for the system alone without them.  The foam, was so bad on all 4 that the cones were barely supported by anything.  I bought 4 surrounds off ebay though.  I cleaned off the old foam with a razor blade, and glued the new ones on the elmers white glue.  You have to press the cone in a little several times as they dry to keep checking that you don't here any rubbing of the coil.  They sound incredible now though!

Austin 

The midrange speakers on the Pentas are another known problem.  I don't have Pentas though so I can't give any details about replacement, but I'm sure some others will chime in.

 (I'm posting as a seperate reply so it's easier to pick out individual speaker models)
 

I think you could safely include the large woofers on many of the older beovox speakers.  The foam on the woofers of my 3702 beovox's are rotted, but I haven't had time to replace them yet.

 So as of now I guess we have.....

Red Line 60

CX-100 series

Penta midranges

older Beovox woofers
 

edit: I had accidentally said my 3800 woofers were rotted too, but they aren't, and don't even have foam surrounds.

It's a question of age AND where the speakers are located. They dont like SUNLIGHT.

I have 3 pairs of pentas and several pairs of redlines, they are all intact, no damage at all. Only one redline 60 was exposed to direct sunlight for several years, this one is damaged by foam rot. Even after repair six or seven years ago, this one has some damage on the foam again. So i think, the main reason for this foam rot is 99% sunlight.

I also found some defect speakers on the MX3500 and MX5500 series, sunlight too.

Martin 

Beolab 6000 drivers have been known to fail. The suspension splits, although they seem to be rubber surrounds.

I recall reading somewhere that humidity plays a part in the deterioration of foam products, as well as UV.

Dillen also supplies Capacitor kits for the RL60 crossovers and offered to refoam the ABR's if you still have the surround and the metal plate.

I have repaired two pairs of RL60's with the replacement "ported plate" from B&O and they do sound fine. (I also have a spare pair of plates for RL60 brand new from B&O unused and still in the box they arrived in)

You can also add that the older Beovox range seem to be fine, the Beovox 5000, 4000, 3000, 2400 etc.

The S45 range, including the S30,60,75 all seem ok, but the slightly later uniphase that culminate in the M150's are also pretty bad for foam rot.

The Beovox 5700 are ok, but the others in that range, such as 4702, 3702 etc have some kind of rubber that just rots.

The ones using foam surrounds are vulnerable, as mentioned the CX100s, 50 etc, but the Pentas can suffer, along with the Redlines.

One thing that does hasten the decline of the foam surrounds is if the owner is a smoker, the tar causes them to decay, add in a warm environment, such as mounted high up as the redlines often were, and those surrounds rot away.

Damp environments are also hard on the earlier 1960's range of speakers, especially the cone tweeters.The only problem I've had with Beovox 3800s is that the tweeters can be damaged and make the sound very harsh.

 I can't comment on the later Beovox panel speakers.

The foam on my 3702's rotted, with absolutely no exposure to sunlight ever, so I don't think the sunlight is the main cause.

 edit: I had accidentally said my 3800 woofers were rotted too, but they aren't, and don't even have foam surrounds.