Explicit mentions of how magic items are affected on the planes are very rare in the Manual of Planes and they are very specific to individual planes, with no general arguments laid out. So this issue appears to be in the purview of individual DMs. Those who are familiar with 2e Planescape rules would tend to apply the rules of magic to magical effects from all sources, including the magic items. [Planescape Campaign Set, A DM Guide to the Planes, page 15: "Most magical items are bound by the same rules and limitations as wizard spells when it comes to functioning on the planes."]
Yet, if we want to stick to 3/3.5e RAW, we could decide to focus on whatever the text explicitly states. The key wording is "difficult to cast". If an effect is not being cast, then we can rule that it is not necessarily impeded. Yet if we are using items to cast spell or spell-like abilities, then they would be affected.
So let us focus on the section of the SRD that pertains to the use of magic items. I would argue that two kinds of magic item activation is some form of casting:
- Spell completion: Typical examples of this type of activation are scrolls. The description of the activity makes frequent use of the words casting, spellcasting, spellcaster, etc. In particular, spell completion is stated to be the act of "finishing parts of the spellcasting". These kinds of items are the ones most likely to be subject to the rules of impeded casting.
- Spell trigger: The activation of these items is "similar to spell completion" and requires "special knowledge of spellcasting". I would say the use of these items would likely be impeded as well. Typical examples of items that get activated by the spell trigger method are wands, in whose description we even have an explicit statement: "imbued with the power to cast a specific spell". Similarly, staffs are spell trigger items, and we can read the words "casting a spell from a staff" in their SRD discussion.
As a personal comment: In our table, we would probably consider assigning some circumstance bonus to the spellcraft check when casting spells from items activated with the spell trigger method.