I've done some checking on the definition of "attack" in the PHB: p269 draws it up in quite some detail, specifying that it requires an attack roll on several occasions. This does mean that if a source of damage doesn't rely on an attack roll, that it doesn't count as an attack.
There are 3 basic positions:
1) As outlined above, a source of damage doesn't count as the effect of an attack unless it requires an attack roll at some point. This is RAW, I believe, based on p269.
[This would allow Wall of Fire, Wall of Ice, and numerous other sources to affect someone in a Seal of Binding.]
2) You could say that any source of damage that is derived from an Attack power (even if that power doesn't require an attack roll) counts as the effect of an attack. I don't believe the RAW ever define things this way, but it is a reasonable position to infer that it is what is intended.
[This would allow Slashing Wake, Aura damages, and probably one or two other things to affect someone in a Seal of Binding.]
3) Lastly, you could say that any source of damage, even if it is not based on attack power and does not require an attack roll, counts as the effect of an attack. This may well be the common sense position (barring counterintuitive examples), but is definitely not RAW. It is, however, certainly a reasonable decision for a work-a-day GM to make.
[This would allow nothing at all to affect someone in a Seal of Binding.]
Basically, if you view the RAW as taking anything other than position 3, then there are ways to stack damage on top of a Seal of Binding, making the inevitability of death more certain.
(I know it was only a secondary point, but I got bored...

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