> I'm considering throwing away my old 56k and switching to DSL, [...]

> [...] one big problem: it can have (and has) only one network card.
> I know that what I want to accomplish is possible using interface
> aliases, but my question is, are there any serious drawbacks in doing
> that?

There are drawbacks.  How serious they are, you will have to judge that
for yourself.

As someone (Joel Rees, the headers say) said, one issue is speed,
because everything from the outside will have to pass into that
inteface and then back out again, all in the same collision domain.

> I remember having read somewhere that having a router with only one
> network interface is not secure -- is that true?

Yes and no.

Things are not "secure" or "not secure" - or at least, to the extent
that they are, nothing is "secure".

The first question I'd ask is "what is your threat model?".  That is,
what threats are you trying to defend against?  Also important, as Joel
pointed out, is how your uplink equipment (DSL modem in your case)
behaves when it sees multiple MAC addresses connected directly to it.

If the modem can be told to pay attention to only one of them, you are
probably OK.  But if not, you are at risk of exposing your internal
traffic to the outside world, with all the consequences that entails.

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