iwmtv: I know you mentioned that you want something that works over 15-30 minutes. But if 9 minutes would work for you, maybe check out the
INSTEON SwitchLinc V2 Dimmer, this can be programmed for up to a 9 minute ramp rate. However, you do need a separate controller / programmer for this.
I'm not sure what the brightness resolution of the X10 dimmers are, but I'm thinking it's not high enough to simulate a smooth increase in intensity. It would probably be more of a noticeable jump every few minutes (based on your macro). As
paulnoel12 stated, you would need software (I.E.
ActiveHomePro) to control this.
Based on the all thats involved with setting up an X10 system and the possibly that the result would not be what you want, it may be cheaper & better to stick with the dedicated simulator like the one you showed me in a PM (
Light Therapy Products: Dawn Simulator).
Of course, if you already have an X10 set-up and / or planning on one anyways, then give
ActiveHomePro and a macro a try first.