Luc:
These are symbols which should be visible under IE6, but not under IE7.
The last column uses my scripting solution: all rows should be correct in any browser (though if you look carefully, the font will differ as between IE and others).
The downside of the scripting solution is that you may need to explicitly allow scripts, in both browsers. This is true in IE7, and will be true in FF if you're running, eg, the "noscript" extension.

Name Entity Ref Hex ID ref Symbol font ref Java Scripted
Rightwards double arrow ⇒ ⇒ Þ
For all ∀ ∀ "
There exists ∃ ∃ $
Nabla ∇ ∇ Ñ
Element of ∈ ∈ Î
Contains as member ∋ ∋ '
Proportional to ∝ ∝ µ
Angle ∠ ∠ Ð
Logical and ∧ ∧ Ù
Logical or ∨ ∨ Ú
Cup ∪ ∪ È
Therefore ∴ ∴ \
Similar to ∼ ∼ ~
subset of ⊂ ⊂ Ì
superset of ⊃ ⊃ É
subset-eq ⊆ ⊆ Í
superset-eq ⊇ ⊇ Ê
oplus ⊕ ⊕ Å
up tack / perp ⊥ ⊥ ^

For what it's worth, IE7 appears to have made visible the following characters, which weren't in IE6: left ceiling (8968); right ceiling (8969); left floor (8970); right floor (8971); plus the following Greek-language glyphs: Greek small letter theta symbol (977); Greek upsilon with hook symbol (978); Greek pi symbol (982). (Ids in decimal; all are visible in FF).

Here is a link to Ian Hutchinson's math-intensive lecture notes, processed by tth . Can you see the "nabla" and "logical and" symbols, in, eg, eq. 2.25? They are produced using the symbol font.